Scoliodon ( Shark) is a poikilothermic animal. It is a cartilaginous fish. Frog ( Rana) is a cold blooded and amphibious animal. The circulation of blood is carried by closed vessels. The vessels which supply blood to the various organs of the body are known as arteries as the net work of arteries form the Arterial system. The walls of arteries are thick and lumen is narrow. The blood pressure is high in the arteries. Arteries do not possess valves. The arteries end in capillaries. Arteries deeply seated in the body. Mostly arteries contain oxygenated blood. A few arteries also carry deoxygenated blood to the respiratory organs (either gills or lungs) for purification.
Scoliodon (Fish) Rana (Frog)
1. The arterial system consists of a ventral aorta, afferent and efferent branchials, dorsal aorta, and its branches and hypobranchials. 1. The arterial system consists of a truncus arteriosus, three pairs of aortic arches and the dorsal aorta & its branches.
2. Five pairs afferent branchial arteries are present. 2. Absent.
3. Efferent branchial system is associated with gill-slits along with the respective arteries. 3. Absent.
4. The arteries to the head are given off from the first pair of epibranchials and by the branches of dorsal aorta carotid labyrinth is absent. 4.The head is supplied blood by the branches. Carotid arteries arising from the truncus arteriosus. Carotid labyrinth is present.
5. Parietal arteries are present. 5. Parietal arteries are absent.
6. Hypobranchial plexus is present. 6. It is absent.
7. Dorsal aorta is formed by the union of epibranchial arteries of both the right and left sides. 7. The second branches of turncus, the systemic arches of the left and right sides unite to form the dorsal aorta.
8. Subclavian arteries arise from the dorsal aorta. 8. Sub clavian artery arises from each systemic arch.
9. Absent. 9. Occipito-vertebral artery arises from the systemic arch of each side.
10. Coeliaco-mesenteric artery aris­es from behind the union of the four pairs of epibranchials. 10. Coeliaco-mesenteric artery arises from the junction of the two system¬ic arches.
11. Just below the Coeliaco-mesen­teric artery, lienogastric artery arises. 11. Lie no gastric artery is absent.
12. The parietal artery gives off four pairs renal arteries. 12. Four pairs of renal arteries arise directly from the dorsal aorta.
13. Gonadial (Spermatic or ovari­an) artery arises from the lieno ­gastric artery. 13. Gonadial arteries arise directly from the first pair of renal arteries.
14. Dorsal aorta terminates into caudal artery. 14. C-iudal artery is absent.
15. Pulmo cutaneous arch is absent. 15. The third branch of truncus is the pulmo-cutaneous arch which is divided into pulmonary and cutanecious arteries.
The heart of fish possess venous blood and blood passes through the heart only once in a complete circuit. But in frog the heart receives both oxygenated and venous blood and the circulation is bi circuit.
The fish is an aquatic animal and possesses five pairs of gills. The blood is supplied by pairs of afferent bronchial arteries and is collected by nine pairs of efferent bronchial arteries. In frog however, the respiratory organs are a pair of lungs (skin & buccal cavity also help in respiration) which are supplied by a pair of pulmonary arteries.
Calotes is a poikilothermic terrestrial lizard. Columba is pigeon adapted for aerial mode of life. Oryctolagus is an herbivorous mammal.
Both pigeon and Rabbit are warm blooded animals. Heart, arteries, veins and blood capillaries are included in the circulatory system. The blood circulation is controlled by an important organ Heart. Normally the blood flows in the closed vessels. So blood circulation is of closed type in verte­brates. The heart possesses auricles and ventricles. The pericardium is at­tached to the heart by gubernaculum cordis'.
The number of chambers of heart varies from calotes and other two vertebrates (Pigeon & Rabbit). The heart contracts and relaxes rhythmi­cally. This is called heart beat.
The detailed comparison of the heart of the above three animals is mentioned below.
Calotes (Lizard) Columba (Pigeon) Oryctolagus (Rabbit)
1. Heart is situated mid ventrally in the an­terior part of the body cavity in the pleuro peritoneal cavity be­hind the sternum. 1. Same way the heart is located. 1. Heart is situated in the thoracic cavity, between the lungs of two sides (Mediastinum). It is present slightly towards the left side.
2. Heart is comparatively smaller in size. 2. Heart is comparatively larger in size. 2. Heart is comparatively larger in size.
3. It is enclosed by double walled pericardium. 3. It is also enclosed in the double walled pericardium. 3. Same.
4. Heart includes a dorsal sinusvenusus aright auricle, a left auricle and a single incom-pletely divided ventricle. 4. Heart is four chambered, sinus venosus is absent in the adult. Completed divided two auricles and two ventricles by inter auricular septum and inter ventricular septum respectively. 4. Same as in columba.
5. The three vena cavae or two precavals and a post caval vein open into the sinus venosus. 5. The three vena cavae or two precavals and a post caval empty the blood directly into the right auricle. 5. Same as in pigeon.
6. The left auricle receives two pulmonary veins from the lungs. 6. The left auricle receives four pulmonary veins from the lungs. 6. Left auricle receives two pulmonary veins from the lungs.
7. The right auricle pos­sess sinu auricular aperture guarded by valve. 7. Absent. 7. Absent.
8. The two auricles are completely separated by inter auricular sep­tum. But the inter ven­tricular septum in the ventricle is incomplete. Hence oxygenated and deoxygenated types of blood is mixed to some extent in the ventricle. 8. Complete inter auricular and inter ventricular septa are present. There is no possibility of mixing the oxygenated blood with deoxygenated blood. 8. Same as in pigeon.
9. The heart of lizard is in a transitional stage approcarhing the double circuit stage But it has not reached it completely due to incomplete division of the encircle. 9. The heart is a double circuit heart because of complete division of ventricle into right and left chambers. 9. Same as in pigeon.
10. The auriculo ventricular aperture is guarded by two flap like semilumar valves. 10. The right auriculo ventricular aperture is guarded by two large muscular flap like valve and the left by three valves. 10. The right auriculo-uentricular aperture is guarded by tricuspid valve and the left by bicuspid valve (mytral valve).

11. There are three aortic arches arising from the ventricle.

  1. Pulmonary trunk (ventral most)
  2. Right systemic trunk (arise from left side of ventricle)

11. Only two aortic arches originate from the ventricles.

  1. Pulmonary trunk (from right ventricle)
  2. Right systemic trunk (from left ventricle)

i.e. Right aortic arch is characteristic of birds.

11. Only two aortic arches arise from the ventricles.

  1. Pulmonary arch (right ventricle)
  2. Left systemic aorta (Left aortic arch from the left ventricle). Right aortic arch is absent.
12. Ductus caroticus is present (connection between carotid & systemic arches) 12. Absent 12. Absent
13. Lizard's heart presents a transitional heart, since it approaches the double circuit heart but has not yet completely attained. So the heart is less efficient. 13. Avian heart has at tained maximum com olexity and is a double circuit heart, i.e. venous blood is com pletely separated frorr oxygenated blood. 13. Same as in Pigeon.
14. Absent. 14. Sinu-Auricular Node and Auriculo ventricular node are present. 14. SA - node and A.V. node are present. In addition bundle of His muscles are also develop.
Scoliodon is a poikilothermc and cartilagenous fish. Rana is also poikilothermic and amphibious animal. In the circulatory system the heart is the most important organ. The blood vascular system in the vertebrates is of closed type. The heart lies in the pericardial cavity of the coelom. It is on the ventral side of the alimentary canal and present anteriorly. In scoliodon the heart is two chambered where as in Rana it is three chambered.
Heart is a pumping organ of blood. From various parts of the body it collect blood mainly through veins and supplies blood through arteries.
Normally the heart is enclosed by a double walled pericardium which possess pericar­dial fluid. The heart contracts and relaxes rhythmically which facilitate the circulation of blood.
1. Heart is approximately pear-shaped. 1. Heart is approximately pear-shaped.
2. The pericardial cavity is not wide and the pericardium forms double membrane around the heart. 2. The pericardial cavity is not wide and the pericardium forms double membrane around the heart.
3. The heart is formed of a dorsally placed sinus venosus and ventrally placed two auricle, a ventricle and truncus arteriosus or conus arteriosus. 3. The heart is formed of a dorsally placed sinus venosus and ventrally placed two auricle, a ventricle and truncus arteriosus or conus arteriosus.
4. The atrium or auricle is two-chambered and lies anterior to the ventricle. Auricles are separated by Inter auricular septum. 4. The atrium or auricle is two-chambered and lies anterior to the ventricle. Auricles are separated by Inter auricular septum.
5. The auriculo-ventricular valve is membranous. 5. The auriculo-ventricular valve is membranous.
6. The conus arteiiosus is incompletely divided by the spiral valve laterally into cavurn aorticum leading to carotid and systemic arches and the cavum pulmocutaneum leading to the pulmocutaneous arch. 6. The conus arteiiosus is incompletely divided by the spiral valve laterally into cavurn aorticum leading to carotid and systemic arches and the cavum pulmocutaneum leading to the pulmocutaneous arch.
7. The opening of the truncus with valves are arrenged in two transvarse rows. 7. The opening of the truncus with the ventricle is guarded by three semilunar valves arranged in a single row. They devide rruncus into a proximal pylangium and a distal synangium.
8. The walls of the auricle are thick. 8. The muscular walls of the auricle are thin.
9. The walls of the ventricle are highly muscular. 9. Same type of ventricle is present.
10. The lips of the bilaminate valves are connected to the inner surface of the ventricle is prominent part of the heart. 10. The membranous valves are connected to the inner surface of the ventricle by chordae-tendinae. Both auricles and ventricle are essential parts of the heart.
11. The fish heart is venous or branchial heart because it receives deoxygenated blood only. 11. The frog's heart receives both oxygenated and deoxygenated blood. The deoxygenated blood remain separate in the auricles but get mixed in the ventricle.
12. Blood passes only once through the heart in a complete circuit. 12. Blood passes through the heart twice in a complete circuit.
13. Such type of arrangement is absent. 13. The sinus venosus opens into the right auricle through simi-auricular aperture guarded by simi auricular valve which is also known as pace maker.
14. No separate vessel collects oxygenated blood since the heart is venous heart 14. The oxygenated blood is collected by pulmonary vein from lungs and carries into left auricle.
The Appendicular skeleton is one of the divisions of the endo skeleton. It includes the pectoral and pelvic girdles and limb bones. The skeleton of the limb in all the tetrapods shows a similar fundamental and structural similarity. However the differences such as arms, legs, wings and paddles are seen in the respective animals. A few tetrapods have completely lost one or both pairs of appendages. The limbs are totally absent in caecilians, most snakes and snake-like lizards.The Appendicular skeleton is one of the divisions of the endo skeleton. It includes the pectoral and pelvic girdles and limb bones. The skeleton of the limb in all the tetrapods shows a similar fundamental and structural similarity. However the differences such as arms, legs, wings and paddles are seen in the respective animals. A few tetrapods have completely lost one or both pairs of appendages. The limbs are totally absent in caecilians, most snakes and snake-like lizards.

The typical tetrapod hind limb can be divided into three seg­ments. The thigh, shank and foot (pes) are the three segments. If there are five toes, normally this type of limb is known as pent dactyl limb.
The skeletal structures of the hind-limb consists of femur, tibia, fibula, tarsals, metatarsals and phalanges.

The femur is the bone of the high and its head articulates with the acetabulum. Its distal end articulates with fibula. The tibia and fibula are the bones of the shank region. They articulate with femur proximally and distally with the tarsal’s of the ankle bones. The fibula bears the most of the body weight.

The foot can be divided into ankle, instep and toes. The ankle is supported by tarsals, which are arranged in rows. The skeleton of ankle or tarsus is the most stable of the regions of the ankle. The instep or metatarsus is supported by the metatarsals. These are elongated bones. The metatar­sals are followed by linear series of phalanges of the toes. The phalanges number varies from 1 to 5.

The first toe of the hind limb is called 'hallux or great toe' and the fifth toe is the 'minimus'.
Calotes (Garden Lizard) Columba (Pigeon) Oryctolagus (Rabbit)
1. The bones of the hind limb are femur, Tibia, fibula, tarsals, metatarsals and phalanges. 1. The bones of the hind limb are femur, tibia, fibula, tibiotarsus, tarsometatarsals and phalanges. 1. The bones of the hind limb are femur, tibia fibula tarsals, meta tarsals and phalanges.
2. The femur is stout bone of the thigh region. It has long, slender and curved shaft in the middle. The shaft enlarges at both the ends. 2. The femur is a stout bone of the thigh region. It has a long, curved shaft in the middle. The shaft has broad ends. 2. Femur consists of long, stout curved shaft. The femur gives support to the thigh region.
3. The proximal end of the shaft bears a rounded smooth head which fits info the acetabulum. There are also distinct prominences lesser trochanter and greater trochanter near the head. 3. The proximal end of femur is produced into a rounded head for the articulation with the acetabulum. Opposite to the head a small protuberance greater trochanter is present. 3. The proximal end of femur bears a rounded knob-like head which fits into the acetabulum. There are three rough projections greater, lesser and third trochanters present near the head. Lesser trochanter lies behind the head, greater trochanter in the middle line and the third trochanter opposite to the head are seen.
4. It is absent. 4. There is an articular surface is present between the head and trochanter for the antitrochanter of ilium. 4. It is absent.
5. Two knob-like condyles are present at the distal end of thefemur. These articulate with the tibia of the shank. Intercondylar groove is present between the two condyles. Patella is absent. 5. The distal end of femu has two prominent condyls with a intercondylar groove. Patella slides in the intercondylar groove on the anterior side. It is a disc-like sesmoid bone. 5. The distal end of femur is pulley-like having two condylesfor tibio-fibule which are separated by a patellar groove. A large sesmoid bone called the patella slides in the patellar groove. It is attached to the tibia by a ligament. Patella is present at the knee-joint.
6. The shank consists of two long bones - the tibia and the fibula. They are separate bones. 6. Tibiotarsus fibula is formed of tibiotarsus and fibula. They are separate bones. 6. Tibiofibula is formed of tibia and fibula. They are separate bones.
7. Tibia is a stout and curved bone present on the inner side. Its proximal end bears two concave facets for the articulation with the femur. It has also a longitudinal ridge the cnemial crest on the side. Tibiotarsus is absent. 7. Tibiotarsus is a large straight and stout bone and also longer than fibula. It is formed by the fusion of tibia and proximal row of tarsals. The proximal end of it bears a pair of articular surfaces for the condyles of the femur and in between them the cnemial crest for the attachment of tendon of extensor muscles. 7. Tibia is stouter towards the anterior end and narrow towards the posterior end. Its proximal end bears two concave facets for the articulation with the femur and distinct cnemial crest on side.
8. Tibia distally bears a concavity for the tarsals. 8. Tibio tarsus distally bears a pulley-like articular surface for the tarsals which is surrounded by a pair of distal lateral tubercles. 8. Tibia distally bears articular surface for the tarsals.
9. Fibula is a slender bone present on the outerside. It bears facets on either side. 9. Fibula is small, slender bone. It is closely applied to the tibiotarsus. 9. Fibula is a slender and weak bone. It lies on the outer side. The bone is narrower towards the distal end and is closely applied to the tibia.
10. Tarsab are five in number which are arranged in two rows. Proximal row has two tarsals the larger compound piece formed by the fusion of a rjbiale, intermedium and centrale and present infront of tibia. A small fibulare present infront of the Sbula. The distal row has three small tarsab called distal tarsab or distalia. 10. The free tarsals are absent. The proximal row of tarsals are fused with tibia and forms tibiotarsus. The distal row of tarsals are fused with the metatarsals and forms tarso metatarsus. It is as long as the femur bone. It is straight and stout. 10. There are six tarsal bones which are arranged in two rows. The proximal row tarsab are two, astragalus and calcaneum. Astragalus is considered to represent two fused tarsals. Calcaneun is produced back wards into a strong calcaneal process which forms the heel. The central row has only one tarsal-centrale or navicular. The distal row contains three tarsab. The first distal tarsal is absent due to the absence of hallux. The second distal tarsal is mesocuneiform which is the smallest distal tarsal. The third distal tarsal is ecto cuneiform which largest one. The fifth distal tarsals are fused to form largest bone in the row - cuboid.
11. There are five meta-tarsals corresponding to the five toes. 11. There are four meta tarsals. The first one is free and in the form of a small projection. The second, third and fourth are fused with the distal row of tarsals to form tarso metatar­sus. Ankle joint is known as mesotarsal. 11. There are four meta tarsals. There are second, third, fourth and fifth, meta tarsals. The first one is absent
12. There are five toes. There are two pha­langes in the hallux, three in the second, four in the third, five in the fourth and three in fifth toes. The pha­langes formula can be expressed as 2, 3, 4, 5, 3 (same as for the hand). The terminal phalanx of each toe supports a strong, curved, horny & pointed claw. 12. There are four toes. The hallux is directed backwards and contain two phalanges. The second toe with three, third one with four and the fourth one with five phalanges are formed. The phalanges formulae can be ex¬pressed 2, 3, 4, 5. The terminal phalanx of each toe is pointed and curved which supports a strong, pointed horny claw. 12. There are four toes. Each toe has three phalanges. The phalanges for­mula can be ex­pressed as 3, 3, 3, 3. The terminal part of each phalanx is pointed and curved to support a horny claw.
The Appendicular skeleton is one of the divisions of the endo skeleton. It includes the pelvic and pectoral  girdles and limb bones. The skeleton of the limb in all the tetra pods shows a fundamental and structural similarity. However, the differences such as arms, legs, wings and paddles are seen in the respective animals. A few tetra pods have completely lost one or both pairs of appendages. The limbs are totally absent in caecilians, most snakes and snake-like lizards. In sirens, the lizard-chirotes, manatees and dugongs only fore-limbs are present. The Appendicular skeleton is one of the divisions of the endo skeleton. It includes the pelvic and pectoral  girdles and limb bones. The skeleton of the limb in all the tetra pods shows a fundamental and structural similarity. However, the differences such as arms, legs, wings and paddles are seen in the respective animals. A few tetra pods have completely lost one or both pairs of appendages. The limbs are totally absent in caecilians, most snakes and snake-like lizards. In sirens, the lizard-chirotes, manatees and dugongs only fore-limbs are present.

A typical tetrapod fore limb can be divided into three segments. The upper arm, fore arm and hand (menus) are the three segments. As there are five fingers normally, this type of limb is known as pentadactyl limb.
The skeletal structures of the fore limb consists of humerus, radius ulna, carpals, Meta carpals and phalanges.

The humerus is the bone of the upper arm and its head articulates with the glenoid cavity .Its distal end articulates with the ulna The Radius and ulna are the bones of the fore arm. They articulate with humerus proximally and distally with the carpals of the mist bones. The radius bears most of the body weight.

The hand can be divided into wrist, palm and digits (fingers). The wrist is supported by carpal bones which are arranged in rows. The palm is supported by the metacarpals. The metacarpals are followed by linear series of phalanges of the fingers The phalanges number vary from 1 to 5.

The first finger of the fore limb is called 'pollex or thumb' and the fifth finger is the 'minimus'.
Calotes (Garden Lizard) Columba (Pigeon) Oryctolagus (Rabbit)
1. The bones of the fore limbs are humerus, radius, ulna, carpals, metacarpals and phalanges. 1. The bones of the fore limb are humerus, radius, ulna, carpals carpometa carpus and phalanges. 1. The bones of the fore limb are humerus, radius, ulna, carpals, metacarpals and phalanges.
2. Humerus is in the form of a long bone with proximal and distal ends. 2. Humerus is a long & slightly flattened with a bent shaft associated by proximal and distal ends. 2. Humerus possess a proximal head, shaft and a distal end.
3. The proximal end of humerus is round and distal end is pulley like with two articular surfaces for the radius and ulna. Supra trochlear foramen is absent. 3. The proximal end of humerus is highly expanded and form into the head A prominent deitcid ridge and a pneumatic foramen are present near the head. The distal end articulates with the radius and ulna by the articular surfaces. Supra trochlear foramen is absent. 3 The proximal end of hu-merus is divided into two parts by a bicipital groove. One part has head which fits into the glenoid cavity. This part has lesser tuberosity. The greater tuberosity is present on the other part. Shaft is present along with deltoid ridge. The distal end has median and lat¬eral epicondyles. Pulley-like trochlea is formed at the distal end which articulates with ulna. Suprotrochlear foramen is present.
4. Two elongated and separate radius and ulna bones are present. 4. Same as in calotes. 4. Same as in columba.
5. Radius is a slender bone. It has a styloid process and concavity for the carpalsdistally. 5. Radius is a straight and slender bone. It has a concavity for the articulation with humerus at the proxima end The distal end is convex. 5. Radius is small slen­der and slightly curved bone With a concavity at the proximal end. The distal end is flat.
6 Ulna is rod liket and stoutet than radius, Proximally it has ole cranon process to articulate with humerus. Distally it has a concavity for the articulation with carpals. 6. Ulna is stouter and longer than radius It is slightly curved. A cranon process to blunt olecranon process is present at the proximal end. The distal ends of radius & ulna articulate with carpometacarpus. 6. Ulna is a long and curved bone. Proximally it bears olecranon process and sigmoid notch for the articulation with the trochlear end of humerus. Epiphyses are present at the distal ends of radius & ulna for the articulation with carpals.
7. Wrist or carpus has ten (10) small bony carpals arranged in three rows. The proximal row has three carpals - radiale, intermedium and ulnare. A centrale lies in the second row. A pisi­form is attached to the distal end of the ulna on its post axi­al side as an addi­tional bone. The third row has five distal carpals. Ex­cept the fourth, the remaining distal carpals are very small. 7. The wrist contains only two proximal carpals. One smaller-radiate and a larger ulnare articulate with radius & ulna respec­tively. The three dis­tal carpals are fused with the meta carpa­ls to form the carpometa carpus. It is a characteristic feature of aves. 7. Wrist consists of eight small carpal bones arranged in two rows. The proximal row contains three carpals-radiale or scaphoid, intermedium or semilunar and ulnare or unciform. The median row has a single centrale. The distal row comprises four true carpals-trapezium, trapezoid, smallest magnum and largest unciform.
8. Carpometa carpus is absent Five slen­der meta carpals support the palm. These are of unequal size & with expanded ends. The middle or third meta carpal is the longest, the second and fourth are only a little shorter than the third. The first and fifth meta carpals are much shorter. 8. Meta carpals are three in number which are fused with the distal carpals and form an elongated compound bone carpometa carpus. 8. There are five long, slender and of unequal size metacarpals support the palm. The first is the shortest and the third is the longest. Each meta carpal has small epiphysis at their end with a middle slender shaft. Carpometa carpus is absent.
9. There are five fingers. 9. There are three fingers. 3. There are five fin­gers.
10. The phalanges are the small bones sup­port the fingers. The number of phalanges differ in the respec­tive fingers. The first finger has two, sec­ond has three, third has four, fourth has five and fifth has three phalanges Thus the phalanges formula can be ex­pressed as 2,3, 4, 5, 3. 10. The phalanges are the small bones sup­port the fingers. The first finger has one, second has two and third has one phalan­ges. Thus the phalan­ges formula can be expressed as 1, 2, 1.There are no claws on the fingers. 10. The phalanges are small bones and their total number is 14. The first finger has two phalanges & the remaining four fingers have three phalanges each. Thus the phalanges formula can be expressed as 2,3,3,3.3.
11. Sesamoid bones are absent. The distai phalanx of each finger supports a strong curved, pointed claw is formed from the epidermis. 11. Sesamoid bones are absent. 11. Sroas nodule-like bones are present on the underside of the fingers. These are seen at the joints between the meta carpals and the first phalanges and also between the second and third phalanges. These provide additional strength to the fingers during burrowing.
12. It is a penta dactyl limb. 12. The fore limb supports the wing. 12. it is a penta dactyl limb.

The hard parts of the animal body are collectively known as skeletal system or simply skeleton. The vertebrates possess the hard parts inside the body. It is known as endo skeleton. The endo skeletal structures are formed with cartilages and bones which are the living tissues. The endo skeleton has been divided into: The hard parts of the animal body are collectively known as skeletal system or simply skeleton. The vertebrates possess the hard parts inside the body. It is known as endo skeleton.

The pelvic girdle is directly attached to the vertebral column in the sacral region. The pelvic girdle consists of two similar halves which are known as ossa innominata. Each os innominatum is. formed by three bones. The dorsal bone is known as ilium, antero-Ventral bone is named as pubis and the ventral bone is called ischium. The pelvic girdle has a depression (concavity) at the junction of the three bones. It is known as acetabulum, into which the head of femur of the hind limb articulates.
bird pelvic girdle18
The same bones are present in all the pelvic girdles of the different vertebrates but have undergone modification.
rabbit pelvic girdle16
Calotes (Garden Lizard) Columba (Pigeon) Oryctolagus (Rabbit)
1. Pelvic girdle is stout and solid. Ifts well suited for walking habits. 1. Pelvic girdle is large and pneumatic. It is well suited for bipedal locomotion. 1. Pelvic girdle is stout and associates with the vertebral column. It is adopted for swift running.
2. Each os innominatum is formed by the ilium, ischium and pubis. 2. Same as in calotes. 2. Each os innominatum is formed by ilium, ischium, pubis and cotyloid cartilage bones.
3. The bones are structurally united. 3. The bones are compactly fused. 3. Same as in columba.
4. Ilium is strong, rod shaped and is directed upwards. 4. Ilium is long, thin and plate-like bone. It is differentiated into preacetabular and postacetabular regions. 4. Ilium is large and broad. The antero-dorsal edge is raised to form iliac crest.
5. Ilium articulates with two sacral vertebrae. 5. Ilium articulates with synsacrum. 5. Anteriorly ilium has articular surface for the sacral vertebrae.
6. Ischium is flat, slightly curved and axe-shaped. It is directed downwards and backwards. 6. Ischium a flat bone fused with the post acetabulariiium. They are separated by ilio-ischial foramen. 6. Ischium is broad and slightly curved bone lying behind ilium. It is posterodorsal in position.
7. Ilio-ischial foramen is present. 7. llio-ischial foramen is large. 7. Ilio-ischial foramen is absent.
8. Ischial tuberosity is absent. 8. It is absent is pigeon. 8. Ischium bears an ischial tuberosity.
9. Ischial symphysis is present. 9. It is absent. 9. Ischial symphysis is present.
10. Hypoischium is present between the ends of the two ischia. 10. Absent. 10. Absent.
11. On the ventral side posteriorly the pubis is formed like a flat elongated and slight ly curved bone, pubis. 11. Pubis is long, slender, curved bone. It lies ventral and parallel with ischial, Pubis. 11. Pubis is flat curved bone directed ventrally pubis symphysis is present. Epipubis is absent.
The hip or pelvic girdle' is present in the posterior side of the body to which the pelvic fins or hind limbs are attached. The pelvic girdle is connected directly to the vertebral column in the sacral region. The pelvic girdle has two equal halves which are known as 'ossa innominata'. Each as innominatum is formed by three bones. They are the dorsal bone ilium, the ventral bone-ischium and the antero-ventral bone pubis. The pelvic girdle has a depression at the junction of the three bones. It is termed as acetabulum into which the head femur of the hind limb articulates and forms a ball and socket joint.
frog pelvic girdle15
In the different vertebrates, the same bones are present in the pelvic girdle with some modifications.
Shark (Scoliodon) Frog (Rana)
1. The pelvic girdle is formed with cartilage tissues. 1. The pelvic girdle is formed chiefly with bone tissues.
2. It is embeded in the body wall muscles infront of the cloacal aperture. 2. It is present at the hind end of the trunk.
3. It is a simple transverse bar known as ischio - pubis bar. 3. It consists of two similar halves which are separated infront and fused behind to form a median vertical disc.
4. Each half of the girdle is formed by the fusion of ilium ischium and pubis. 4. Each half of the girdle consists of three bones - ilium, isclium and pubis.
5. Acetabulum is absent. 5. Each side of the vertical disc bears a cup-like depression Acetabulum. The head of femur of the thigh bone articulates with the acetabulum. So all the three bones take part in the formation of the acetabulum.
6. The ilium possess an iliac process and a foramen. 6. The ilium extends forwards in the form of an arm to articulate with the transverse process of the sacral vertebra. A vertical ridge is formed along with this arm is called iliac crest.
7. Ischium and pubis fused together and form Ischio-pubis bar. 7. The ischium forms the posterior part of the disc and acetabulum. Ischium, fuses with the other side ischium and forms ischium symphysis.
8. Pubis fuses with ischium. It is not a separate bone. 8. The pubis forms the ventral part of the disc and acetabulum. It fuses with the pubis of the other half and forms pubic symphysis. It is a separate bone.
9. Pubis is formed with cartilage tissue. 9. Pubis is formed with calcified cartilage tissue.
10. The pelvic girdle is straight in the middle but bent at the ends. These are produced dorso-lat-erally into short iliac processes. 10. The pelvic girdle V-shaped associated with a vertical disc formed with the ischium & pubis bones.
11. The pelvic fins are attached directly. 11. The hind limb bones are articulating with the pelvic girdle.
12. The pelvic girdle provides attachment to the claspers through the muscles of male. 12. Such arrangement is absent. Penis is absent.

15 Main Theories of Biological Evolution of Man (with Statistics)

Read this essay to learn about the 15 main Theories of Biological Evolution of Man !


1. Theory of Eternity:

This is an orthodox theory. It believes that some organisms were there from the very beginning of the Universe. Those organisms still exist and will be continued in future in addition to some new forms. According to this theory, the original forms are eternal, and they have been preserved automatically. But this view is not at all popular; it is held by a few people only.

2. Theory of Divine Creation:

A Spanish Monk, Father Sudrez (1548 – 1617) proposed this theory. It was based on the Biblical book of Genesis. According to Genesis, of Old Testament of Bible, the world was created by the supernatural power (God) in six natural days.
The theory specifies that all creations, including plants, animals and man on earth were created during those six days. Since all species were made individually by god, the theory does not accept the idea of origin of new species from ancestral forms. Life is considered as a vital spirit according to this theory.
The Hebrew and the Christian Church authorities had supported this view for many Centuries. To them, god created Adam and Eve, the two companions of opposite sex about 6,000 years ago, from whom the human beings have descended.
Archbishop Ussher (1581 – 1656) pointed out 4004 BC as the exact year for the creation of man. Each and every followers of this theory believed that all creations of god are arranged in a chain where human is posited at the top.

3. Theory of Spontaneous Origin:

The theory contends that life had originated repeatedly from inanimate materials or non-living things in a spontaneous manner. The concept was held by early Greek philosophers like Thales (624 – 547BC), Empedocles (485 – 425BC), Democritus (460 – 370BC), Aristotle (384 – 322BC) and others.
Aristotle thought that fireflies originated from morning dew and mice from the moist soil spontaneously. All succeeding Greek philosophers and many scientists shared Aristotle’s view till the middle of the seventeenth Century. Louis Pasteur partially accepted this theory.

4. Theory of Catachysm or Catastrophism:

French geologist Georges Cuvier (1769 – 1832) proposed this theory. His observation was based on the fossil remains of varied organisms. According to him, the earth had to face severe natural calamities at different times for which many animal species have been destroyed. But each time when the earth settled after a great Catastrophe, relatively higher forms of animals appeared to replace the situation.
Cuvier did not believe in continuous evolution. To him the species never evolved by modification and re-modification; a series of Catastrophes were responsible behind changes where previous sets of living creatures get replaced by new creatures of complex structure.
As per his scheme, corals, molluscs and crustaceous appeared in the first phase. Then came the first plants being followed by the fish and reptiles. The birds and mammals appeared thereafter and in the last phase man emerged about five to six thousand years ago.

5. Theory of Uniformitarianism:

This theory was presented by Charles Lyell (1797 – 1837) in his work ‘Principles of Geology’. Being a geologist, he could not accept the concept of an unchanging earth. By studying the rocks and geological processes, he came to the conclusion that, at the beginning, some forces were in operation to shape and reshape the earth. Animal forms gradually evolved along with this change. Fossils were the main support for his evidence. This theory on one hand discarded the “theory of Catastrophism” and on the other hand nullified the “theory of divine Creation”.

6. Theory of Cosmic Origin of life:

This theory advocated that the first life seed had been transported through the cosmic particles from other planet. Richter (1865) developed this theory and he was supported by Thomson, Helmholtz (1884), Von Tieghem (1891) and others.
According to them the meteorites that travelled through the earth’s atmosphere, contained embryos and spores in them; those gradually grew and evolved into different types of organisms. But the concept lacked evidences and interplanetary exchange of viable spores and embryos could hardly be possible in the light of current understandings.

7. Theory of Cynogen:

German scientist Fluger proposed this theory. According to him, the cynogen, a complex chemical compound was developed by sudden reaction between the atmospheric nitrogen and carbon. This cynogen later gave rise to first protein substance, which ultimately created life through various types of chemical synthesis.

8. Theory of Chemo-synthesis:

This theory also recognized a complex type of chemical synthesis. It pointed out different kinds of materials, which in varied natural environment produced a large number of actions and interactions. As a consequence, life developed in a peculiar set up following a complicated situation.

9. Theory of Virus

Some scientists believed that virus was initially responsible for the emergence of life. The viruses hold a transitional stage between living and non-living. By nature a virus is non-living, but when it reaches to the body cell of the living host, it behaves as living. Therefore, it was thought that such a creature might possess a role in the emergence of life.

10. Theory of Organic evolution

According to this theory, origin of life must have taken place in this world. First living existence was very minute and in the form of unicellular structure. As the time passed on, most of the unicellular forms were transformed to multicellular forms under the various environmental oscillations. Gradually and gradually simple form of animals was converted to very complex type of animals.
As a matter of fact, the geo-environment of the earth underwent a process of continuous change and influenced the animal forms. Complex forms of animals evolved out of the simple forms in a slow and steady way. This process of change has been designated as organic evolution. The conception of organic evolution maintains its conformity with ancient Hindu religious thought. B.M. Das (1961) wanted to prove this with the example often incarnations of Lord Krishna (Dasha avatar).
He mentioned that the first incarnation was a fish (Matsya avatar). He justified his remark by comparing it with the western belief where the life was thought to be originated in water. The second incarnation according to Das was a turtle (Kurma avatar), an amphibian. The next incarnation was a wild pig (Baraha avatar) which represents land-living animals. The fourth incarnation was a mixed form with half man and half animal (Nrisingha avatar). This idea complies with anthropological outlook.
All of the anthropologists now agree that the stage before true man was a combination of man and ape. However, the fifth one was a short-statured incarnation (Baman avatar). It indicates the fact that early men were short stature.
In this way Prof Das described not only the biological evolution, but the cultural revolution too. He also mentioned that Parasurama was defeated by Rama, as Rama possessed bow and arrow, a superior weapon than the axe. The stage corresponded to the food-gathering stage of prehistory and it was followed by a food-producing stage as depicted in the story of Lord Krishna who used to look after the cattle in his childhood and his elder brother Balaram carried a plough most of the time.
In the Christian era, before Darwin, several scientists and philosophers expressed their views regarding the evolution. In this context, Carl Linnaeus (1707 – 1778) made a classic work “Systema Natural” where he described a system of classification involving the plants and animals, known as taxonomy. 
He placed man in the order Primate along with apes and monkeys, but he did not suggest any common ancestry for them. Further, he believed that each species was created specially and separately; their position remains unchangeable. In this way, the proposition of Linnaeus was a combination of the Old belief and the new thought.
Men boddo (1714-1790) by observing the origin of species traced the evolution of man from the monkeys. Bonnet (1720 – 1793) also worked on the process of evolution and proposed a ‘scale of beings’. His proposition went on an ascending order from the mineral to man. Many more scientists worked with the origin of man. Among them, the contributions of Erasmus, Darwin (1731- 1802), Karl von Baer (1792-1876), Schopenauer (1788 -1860) and Charles Lyell (1797 – 1875) seem to be indispensable for proper understanding of the facts of evolution. Imanuel Kant (1724 – 1804) proposed that the man be descended from the monkey.
According to a group of scholars, the expression of Goethe (1749 – 1832) was so meaningful in respect of evolution that he may be regarded as a predecessor of Charles Darwin. Again, another scientist, Malthus (1766 -1834) kept valuable contribution towards formulating the theory of natural selection. It is justified to trace the history of evolutionary thought from the beginning of nineteenth Century. The first systematic attempt was made by Jean Baptiste Lamarck (1744 – 1829), a French biologist who was an eminent pre-Darwian student of evolution.
His theory was published in 1802 in which he proposed the ‘inheritance of acquired characters’ during the lifetime of the individual. Following Lamarck’s proposition, Charles Darwin and Alfred Russell Wallace jointly proposed the theory of the ‘Origin of Species’ by Natural Selection.
Charles Darwin’s evolutionary theory had its base on the accumulation of small fluctuating variations. He had realized that heredity was an essential factor in the study of evolution, though he did not put much importance to it. August Weismann realized the importance of heredity better than Darwin did.
He emphasized on the ‘continuity- of the germ plasm’ and tried to project the transmission of inherited qualities from generation to generation by the germ cells. Hugo de Vries, one of the re-discoverers of Mendel’s laws of heredity, announced mutation theory of evolution in 1901. He considered mutation (i.e. sudden hereditary changes) as a factor behind evolution.
Natural selection found very little or no place in his mutation theory. But, later the geneticists, biometricians, and palaeontologists revived the faith in natural selection. Of these, the most important development took place in the field of genetics; the natural selection was started to be restudied and reinterpreted by the geneticists. Mention may be made of Theodore Dobzhansky and R.B. Goldschmidt, who laid the foundation for the Neo-Darwinian theory.
The genetic theory of Natural Selection is therefore referred as Neo-Darwinism. R.S. Fisher, J.B.S. Haldane and Sewall Wright made valuable contribution to the statistical analysis of population and secured own position among the principal proponents of Neo-Darwinism. However, the important theories have been discussed in the following pages.

11. Theory of Lamarck (Lamarckism):

The French biologist, Jean Baptiste Lamarck (1744 – 1829) spent his early years in military service but when he was stationed at Monaco, he acquired interest in Botany. He also established himself as a distinguished zoologist. His extensive studies on invertebrates formed a base in zoological classification.
He was the first scholar to recognize the distinction between invertebrates and vertebrates. But Lamarck’s name is usually associated with the ‘theory of inheritance of acquired characters’. Of his several writings, mention must be made about three publications relating to the theory of evolution: Recherches Sur L ‘Organization des Corps Vivant (1802), Philosophic Zoologique (1809) and Historie Naturelle des Animaux sans Vertebrates (1815 – 1822).
Lamarck expressed the fact that the acquired characters could be inherited. His theory, known as Lamarckism was based on two laws:
i. The law of use and disuse of organs, and
ii. The inheritance of acquired characters.
According to Lamarck, a living body is always influenced by the environmental factors and ultimately this phenomenon initiates an adaptation of organism to its surroundings. As per necessity, some parts of the body may be used more and more.
Therefore, those parts tend to show more development or changes in course of time. On the contrary, other parts of the body, which may not be required much, will be weak or demolished due to constant disuse. This change in body structure is reflected in future generations. This means, the characters that are acquired by the use or disuse of different organs can be transmitted to the succeeding generations.
To support his theory Lamarck presented several examples. The most remarkable one is associated with the long neck and high front legs of giraffes. He stated that this animal originally possessed short neck and small front legs.
As an herbivorous animal, the forerunners of modern giraffe were acquainted with grass and the leaves of dwarf trees. But following a sudden scarcity of these plants, giraffes had to stretch out their necks to reach the leaves of the tall trees. This stretching affected the muscles and bones of the neck, which started to be modified with time. Not only had the neck become longer the front legs also increased in size. This phenomenon is nothing but an adaptation to the environment, in the way to survival.
The modified traits were continued in subsequent generations and eventually all the giraffes got very long necks and well-built long front legs. In another example, he mentioned that the ducks are unable to fly because their wings became weak when they stopped flying.
Again, the birds that started to live in an aquatic environment gradually acquired webbed feet through the conquest of survival. Lamarck also cited other examples like limblessness in snake, blindness of moles and certain cave-dwelling forms, aquatic plants with dimorphic leaves (having submerged and aerial leaves), etc. All these changes were held to be cumulative from generation to generation, and also hereditary.
Lamarck’s theory had met criticisms from several angles. Although some of his views were admitted by a few scholars, most of the scholars did not accept his theory. The German scientist August Weismann ridiculed the essence of Lamarckism (inheritance or acquired characters) by his experiments, which involved cutting of tails of mice for over twenty generations.
All tailless mice in all generations (even in the last generation) produced their offspring’s with tails. Therefore he reached to the conclusion that the environmental factors might have an influence on the body cells, but it is not enough to profess a change of reproductive cells.
Characters of an organism would not be inherited unless the change could occur in the reproductive cells. However, the proposition of Weismann is popularly known as ‘Germ-Plasm theory’ as contrary to the theory of Lamarck. According to Weismann the body of an animal is composed of two parts viz. Germplasm (germ cells) and somatoplasm (body cells); only those characters which are located in the germplasm will be inherited by the offspring.
The evidence against Lamarckism was also criticized by others, on the ground that cutting of tail is rather mutilation, in which the animal did not participate actively so some specific cases were required where organisms can actively participate in the activity. In this respect, McDougall (1938) conducted a series of experiments on learning, using white rats. He designed a water tank having two exits, one lighted and the other dark.
The lighted exit received electric shock, while the dark exit did not have any arrangement to receive the electric shock. The white rats were dropped into such an experimental tank, and then trained to escape through the dark exit. A number of trials were required for the rat to learn the way to escape from the dark exit. These trials constituted a measure of the speed of learning.
The trained rats were bred, and their offspring’s were taught the same problem. In this manner, he subjected the rats for experimentation, for forty-five generations. McDougall observed that the number of errors made in learning, the problem decreased progressively from generation after generation. On the basis of this experiment, he concluded that an acquired character (learning or training) is inherited.
Unfortunately, McDougall’s experiments met with severe criticism, mainly because the repetition of similar experiments in other laboratories had failed to produce similar results. They could not control the genetic constitution of the experimental rats.Limitation of various other experiments probably initiated the scholars for seeking evidence in favour of Lamarck. A new school of thought in the name of Neo-Lamarckism soon appeared in the scene, which tried to modify the principles of Lamarck in order to make it acceptable to the students of evolution.
The foremost position was occupied by Giard (1846 – 1908) of France and Cope (1840 – 1897) of America. However, Neo- Lamarckism was based on the idea of adaptation, integrated with direct and casual relationship between structure-function and environment. The difference between the Lamarckism and Neo- Lamarckism was that, Lamarck believed in direct action of the environment, which, he thought was responsible to achieve final perfection of the individual. But Neo-Lamarckism omitted the very idea.
The Neo-Lamarckians argued that a considerable period of time was required for getting the effect of external factors. They also pointed out that if the external factors failed to influence the reproductive cells of the parents, their offspring’s would never inherit any of the modifications.
Rapid progress of science in twentieth Century favoured the growth of ‘genetics’, which supported none of the theories – Lamarckism and Neo-Lamarckism. Still Lamarck deserves appreciation as his proposition helped to open new avenues of thought in the science of evolution.

12. Theory of Darwin (Darwinism):

Charles Robert Darwin (1809 -1882) was born as the fifth son of his parents. He had an elementary schooling in Shrewsbury, England. In childhood he took little interest in studies, but showed great interest in hunting birds and shooting dogs. His father and teacher considered him as ‘a little below average in intelligence’. Although in school, he showed some interest in mathematics and chemistry, but most of his time was spent in watching the habits of birds, collecting insects and minerals.
In 1825, Darwin was sent to Edinburgh to study medicine, but soon he discontinued the course. After this his father wanted him to be prepared for the post of a clergyman, in the Church of England. So Darwin was sent to Cambridge. While studying at Cambridge, he gained friendship with some distinguished men of science, such as, the botanist Dr. Henslow and the geologist Sedgwick. Dr. Henslow’s friendship entirely changed the course of Darwin’s life; he nominated Darwin in the position of a young naturalist for the voyage on H.M.S. Beagle (a ship, in which Charles Darwm sailed around the world).
The voyage on the Beagle started on 27th. Dec. 1831 and Darwin visited many Islands in Atlantic ocean, some of the islands in the Pacific ocean including Galapagos islands, many places on the coasts of South America and finally returned after five years on 2nd. Oct. 1936. While on the Beagle, Darwin took notes on the flora, fauna, and the geology of the places visited; and also made extensive collections of living and fossil minerals. All these constituted the basis for his future publications.
Darwin’s first publication, Journal of Researches (1839) met with immediate success. In October 1838 he accidentally came across Robert Malthus’ essay on population. This essay provided a clue for which Darwin was able to think of the ‘struggle for existence’ among the animals and plant kingdom.
In this respect, he started to collect the data from 1842. The famous geologist of that period, Sir Charles Lyell suggested him to write about the origin of species. In 1858, when Darwin was halfway in his writing, he received a manuscript entitled, “On the tendency of varieties to Depart Indefinitely from the Original type” from Alfred Russell Wallace (1823 – 1913).
Wallace requested Darwin to read his essay and to make comments on it. Darwin found that the essay was complete in all respects and contained the essence of his theory of natural selection. Being generous, he decided to withhold his half-completed work, in favour of Wallace. So he wrote to Lyell with a recommendation to publish Wallace’s paper at once.
But Lyell, being aware of Darwin’s strenuous effort since 1842, urged Darwin to write a short abstract of his theory. He wished that Wallace’s paper would be published simultaneously with Darwin’s abstract. Reluctance of Darwin could not stand against the insistence of Lyell.
Thus, in 1859, Wallace’s paper and an abstract of Darwin’s manuscript together appeared in the Journal of the Proceedings of the Linnean society. To start with, Darwin intended to complete his work in four volumes but subsequently he condensed the work into a single volume, entitled ‘Origin of Species’ which was published in November 1859.
The work of Darwin was submitted fifty years after Lamarck and his theory is commonly known as Darwinism. But, the credit went to both the scholars – Darwin and Wallace; the first systematic as well as comprehensive approach in the perspective of evolutionary development was made by them.
Darwin’s theory of evolution is based upon four main, rather easily understandable postulates, which may be summarized as follows:
1. Prodigality of Nature:
All species have a tendency to produce more and more offspring’s in order to increase the number of population. For example, a salmon produces 28,000,000 eggs in a single season; a single spawning of an Oyster may yield as many as 114,000,000 eggs; a common roundworm (Ascaris lumbricoides) lays about 70,000,000 eggs in a day.
Darwin has even cited examples from slow breeding animals. Elephants appear to be one of the slowest breeders, having a life span of about hundred years. The active breeding age continues from thirty to ninety years, during which a single female may produce six young ones.
Taking this estimation into consideration, Darwin calculated that a single pair of elephants, at this rate of reproduction (provided all the descendants survived and reproduced at the same rate) would produce 19,000,000 elephants after 750 years.
All these examples furnish instances of tremendous reproductive potential among all species of organisms. The basic reason behind this huge production is to ensure the survival. Because, in reality we find that, in spite of the rapid reproductive potential, the size of a given species, in a given area, remains relatively constant.
2. Struggle for Existence:
Above observation led to the conclusion that all the progeny produced by any generation do not complete their life cycle, many of them die during juvenile stages. Darwin therefore proposed his of Struggle for Existence’; the struggle is often generated for the want of enough resource All individuals cannot survive under struggle.
According to Darwin, the Struggle for existence may be of different types. It may be a Struggle to overcome adverse environmental conditions (like cold or drought), or to obtain food from a limited source of supply. It may be a fight for occupying a living pace, or even to escape from the enemies. However, any of these said situations, evidently leads the members of a group towards competition, in order to meet their requirements.
Thus the nature of struggle may be of three types according to the situations:
(i) Intra-specific struggle:
When the members of a same species struggle among themselves, the situation is considered as intra-specific struggle. Such a struggle is usually centered round the consumption.
(ii) Inter-specific struggle:
The individuals from different species also may go on fighting for survival. An individual from one species may hunt another individual of other species as food. For example, tiger hunt goat and deer; cat hunt rat; lizard hunt cockroach and different small insects; and so on. According to Darwin in the animal kingdom, a species often stand as prey to other species, which clearly indicates a struggle for existence. Such happenings have been referred as Inter-specific struggle.
(iii) Environmental struggle:
The environmental struggle is different from the inter-specific or intra-specific struggle. Here individuals irrespective of their species-identity struggle against the environmental hazards like earthquake, flood, drought etc. Those who have greater potentiality for resistance, only they survive.
Darwin believed that the struggle is a continuous phenomenon in the way to survival It is severe among the members of the same species (intra-specific competition), as they depend on identical requirements of life. The inter-specific competition is though very common, but its frequency is lesser than the intra-specific competition.
3. Organic variation:
Darwin observed that variation is a universal phenomenon. Except the identical twins no two organisms are exactly alike. Even the two leaves of a plant or two peas in a pod often show easily recognizable differences. Therefore individuals of a single species must vary from each other.
At times, an entire population may exhibit a definite pattern of variation for which it is distinguished from the rest of the species. Such a population showing definite pattern of variation is often referred to as subspecies. Darwin considered these subspecies as incipient species, and he believed that in course of time, these subspecies would be subjected to further variation to give rise a new species.
Although natural variations are neither advantageous nor disadvantageous to the species concerned but some variations are considered as favourable and others are unfavourable. In fact, the variations in terms of physiological, structural and behavioural traits play very important role for adaptation in the environment. The new variants are produced continuously but when those variants cannot cope up the environment, it is termed as un-favourable variation.
Organisms with un-favourable variation easily get defeated in the struggle for survival and in course of time they become eliminated from the world. On the other hand, the new variants that are capable to adopt the pressure of the environment survive long. The new traits of advantageous characteristics pass on to the future generation.
Darwin recognized two main types of variation in nature, viz. Continuous variation and discontinuous variation. By the term continuous variation he wanted to mean small fluctuations of evolutionary significance. It was held as a force for attaining perfection being selected by nature For example, the long neck of giraffe was evolved out of continuous evolution.
Contrary to this discontinuous variation is mostly large and rare in occurrence. However, they appear suddenly and do not show any graded series. Such discontinuous variations have been regarded as ‘sports’ by Darwin; to which, Hugo de Vries has given the name ‘mutation’, at a later period. In the eye of Darwini discontinuous variation had no evolutionary importance.
Darwin draws the example of Dinosaurs. The enormous size and giant stature of Dinosaurs were the result of discontinuous variation. He found negative mode of natural selection behind the extinction of those Dinosaurs.
4. Natural Selection:
Natural selection is the final outcome of Darwin’s evolutionary thought. Individuals differ from each other because of organic variation, which evidently means that some individuals are better adapted to survive under the existing environmental conditions than others.
In the struggle for existence, the better-adapted individuals possess a better chance of survival than those who are less adapted. The less adapted individuals therefore get eliminated before reaching maturity and thus a large number of individuals die in the struggle for existence.
However, the traits having greater survival value are preserved in the individuals and transmitted to the offspring’s, who are supposed to be the progenitors of the next generation. Darwin called this principle, by which preservation of useful variation is brought about, as natural selection. The same principle (natural selection) has been designated by Herbert Spencer as ‘survival of the fittest’. In the words of Darwin “the expression often used by Spencer, of the survival of the fittest, is more accurate, and is sometimes equally convenient”.
The theme of Darwin’s theory may finally be summed up in the following words: The organisms always struggle to maintain their existence as nature decides the survival of the fittest one. Adaptive traits preserved through natural selection gradually change the characteristics of species and thus evolution occurs.
The theory of the origin of species by natural selection, though is regarded as a major advancement in evolutionary thought, it lacked the knowledge of heredity, which was essential for the understanding of evolutionary studies. It was really unfortunate that Darwin never came across Mendel’s work, who by then invented the basic principles of heredity. Had Darwin come across Mendel and his work, he would not have to write in the last edition of his ‘origin of species’ that “the fundamental principles of heredity are still unknown”.
The human ancestry was discussed by Darwin in his book, ‘The Descent of Man’ which was published in 1871. He said that life ascended from simplest form of minute organisms to the complex forms through different stages of evolution where man is found at the summit.
But, at the time of Darwin very few fossil evidences were discovered; those were insufficient to establish the proposition. This was the first weakness of Darwinism. The second weakness was hidden in the process itself. Darwin wanted to explain heredity by the ‘theory of Pangenesis’, which declared that all parts of the body produce minute particles called pangenes that ultimately get deposited to the sex-cells being carried by blood.
Those particles are further carried to the next generation when fertilization takes place and same kinds of organ, cell, tissue etc. are reproduced. However, the theory of Pangenesis, like the Lamarck’s principle, accounts for the inheritance of acquired characters. But it too was universally discarded for the lack of evidence. The flaws of Darwin were rectified later, after the development of the science of genetics and the rectified theory was known as Neo-Darwinism.

13. Mutation Theory of Hugo De Vries:

Hugo de Vries (1840 – 1935) was a Dutch Botanist, who proposed the third theory of evolution. His ‘mutation theory’ which appeared in 1901, focused attention upon the importance of mutation in evolution. In this theory, de Vries declared that evolution is not a slow and gradual process involving accumulation of numerous small changes by natural selection. Conversely, the evolutionary changes appear suddenly and are a result of large jumps, which he designated as mutation.
The publication of de Vries’ work raised much controversy among the adherents of Darwinism and the mutationists. The early geneticists extended their wide support to de Vries’ theory, mainly because the variations, which they noted in their experiments, conformed to de Vries’ observations but hardly with Darwin’s concept.
Even eminent geneticists like William Bateson, Thomas Hunt Morgan and others were attracted by this mutation theory Mutation theory distinguished heritable variations from environmental variations, which Darwin failed to understand in his ‘Natural Selection’. As a consequence, in the early years of twentieth Century Darwin’s natural selection was totally rejected in explaining the process of evolution.

14. Theory of Gregor Mendel:

The work of Gregor Mendel virtually remained unknown from 1865 to 1900 until it was rediscovered by three geneticists in 1900, Carl Correns, Hugo de Vries and Eric Von Tschermak. The real mechanism of mutation was properly understood through the work of Gregor Mendel and the recent discoveries in the field of molecular biology.
De Vries’ hypothesis on mutation highlighted chromosomal changes, rather than the changes in the gene themselves. So his mutation theory is considered as out modded on the ground that it did not indicate true mutation.
The mutations as understood today are concerned with genes, the discrete units of heredity, which occupy particular loci on the chromosomes. It tells that each gene controls a specific developmental process and responsible for the appearance of specific traits in an organism.
Mendel used the term ‘factor’, when he described his ‘Law of Inheritance’. But in 1900 the term was replaced by the new term ‘gene’ and a new science gradually developed with the name ‘Genetics’ Now It IS known that a gene represents a specific segment of the DNA molecule.
The product of a gene action in many cases, is a protein; and the developmental process in a given organism depends on specific kind of proteins produced under the instruction of a particular set of genes. A mutation in a gene often causes corresponding changes in the protein concerned. If mutation occurs in the gem cells of an organism, the change will be inherited by its offspring.
Therefore, only those mutations that cause changes in the reproductive cells of the organism are of evolutionary significance But the structural changes of chromosomes cannot be undermined because they often bring considerable effects in the evolution as found in many plants and a few animals like Drosophila, crepis etc.
Although the knowledge of genetics brought a revolution in the field of evolution Mendel’s Law of Inheritance’ is fundamental in identifying the nature of the offspring’s. It explained the basic process of heredity.

15. Synthetic Theory of Evolution (Neo-Darwinism):

Darwinism in its original form failed to explain satisfactorily the mechanism of evolution and the origin of new species. The inherent drawbacks in the Darwinian ideas were the lack of clarity as to the sources of variation and the nature of heredity.
In the middle of twentieth Century, Scientists had come to a consensus to employ all sorts of knowledge – genetic, ecological, geographical morphological, palaeontological etc. in order to understand the actual mechanism of evolution. Due importance was given to both mutation and natural selection, among other forces of evolution This led o the emergence of a synthetic theory of evolution, which we also call as Genetical Theory of evolution, or ‘Biological theory of Evolution’.
Some authors namely David J. Merrell (author of ‘Evolution and Genetics’) and Edward O Dodson (author of ‘Evolution: Process and Product’) have called this new theory as Neo-Darwinism. But, George Gaylord Simpson and his followers strongly warned against equating the synthetic theory of evolution with ‘Neo-Darwinism’. Simpson argued that the synthetic theory had no Darwin. It was not only different from Darwin’s; it had drawn its material from a variety of non-Darwinian sources.
After the development of the science of genetics, it has been known mat a population snares a common gene pool. Accordingly, the evolution denotes a change of gene -frequency in the gene pool of a population over certain span of time.
The synthetic theory of evolution does not discard all previous propositions, rather considers them as partially important. Therefore, we find amalgamation of various concepts viz. Natural selection, Mendelian principles, Mutation, population genetics in this theory of evolution. But it is interesting to note that modem genetics does not acknowledge to mutation theory in its original form, as proposed by de Vries. Because that original theory had out- rightly rejected the basic concept, ‘natural selection’ as delivered by Darwin and advocated ‘mutation’ as the sole force of evolution. However, at present evolution appears to be a complex process involving several complex forces.
About one thousand species of fishes are found in marine and fresh water in Pakistan. Majority of these are edible. And very few are examined for their nematode parasites.
Most of marine fishes are included among the group of edible fishes. Some of these including, Scomberomorus guttatus, Pomadasys olivaceum, Pomadasys maculatum, Pomadasys stridens, Otolithus ruber, Sphyraena forsteri, Sphyraena jello, Lates calcarifer and Sillago sihama are popular edible fishes in Pakistan, due to their delicious taste and are full of nourishment such as proteins and vitamins particularly vitamin E and vitamin D.
Neoteny is the retention, by adults in a species, of traits previously seen only in juveniles (pedomorphosis/paedomorphosis), and is a subject studied in the field of developmental biology.

In neoteny, the physiological (or somatic) development of an animal or organism is slowed or delayed.

Ultimately this process results in the retention, in the adults of a species, of juvenile physical characteristics well into maturity.
In vertebrate biology, neoteny is most easily identified when sexually mature, completely viable juveniles or larva are found.
Life history of Polystomella shows alternation of generations (Sexual method of reproduction will alternate with asexual method of reproduction).
Elphidium-Asexual reproduction :
In Elphidium microspheric form takes up asexual reproduction by multiple fission. The nuclei break into many Chromatin bits. They become round. They are covered by bits of cytoplasm. Several amoeboid cells are formed. These amoebulae are liberated into water. In the water these Amoebulae will develop shell. The first chamber is called 'Proloculum'.
This grows into megalospheric form. It contains only single nucleus. This takes up sexual reproduction.
Elphidium Sexual Reproduction :
Megalospheric form performs sexual reproduction, it has a single nucleus in its cytoplasm. It undergoes many divisions. The first division is reductional. Then gametes are formed. Each gamete shows two flagella. These are all similar. They are called isogametes. They show 3 to microns length. These isogametes coming from two different parents will unite and a zygote is formed This union is called isogamy. This zygote will give rise to microspheric form. This form shows many nuclei in its cytoplasm. It takes up asexual reproduction.
Alternation of Generations :
In the lite history of Elphidium the microspheric form will undergo asexual reproduction and produce megalospheric forms. These forms will undergo sexual reproduction and produce microspheric forms. Thus in the life-history micro and megalospheric forms will alternate. This type of alternation of generations is called Metagenesis, because both alternating stages are diploid.
Elphidium is a marine form. It is found creeping on Sea weeds to a depth of 300fathoms
Phylum : Protozoa
Class : Rhizopoda
Order : Foraminifera.
Elphidium is also called 'Polystomella is a 'dimorphic rhizopod'. It is a unicellular microscopic protozoan, and" 1 mm in diameter It is pale yellow in colour. It lives in marine water. The body is covered by a shell. The shell is biconvex. The first formed chamber is proloculum. The shell contains spirally arranged V shaped chambers. Hence it is called "polythalamus or multilocular They overlap one another. These chambers show 'openings' hence it is perforate Through these openings cytoplasm will come out. The cytoplasm is produced into a number reticulopodia. which will form a network. From hinder end of each chamber cytoplasmic processes will develop. They are directed backwards. They are called retral processes'.
The chambers are filled with the cytoplasm.
  1. The cytoplasm contains one or many nuclei.
  2. Contractile vacuole is absent.
  3. Mouth is absent.
  4. Cytoplasm contains food vacuoles. They take up the process of digestion.
Dimorphism: Polystomella exhibits dimorphism. The individual occurs in two distinct forms.
1. Megalosperic form.
2. Microspheric form.
  1. Megalospheric form: Its proloculum is big in size. A single large nucleus is present in one of the chambers. It takes up sexual reproduction.
  2. Microspheric form: Its proloculum is small in size. Many nuclei are present in the cytoplasm. This form reproduces by asexual reproduction.
Locomotion and Nutrition:
Polystomella show slow creeping movements with the help of reticulopodia. It is a holozoic feeder. These Pseudopodia will capture the prey. When it comes in contact with the prey, it kills the prey by secreting toxic substance. The prey is digested in the food vacuole the digested food is absorbed by the cytoplasm.
Protozoa = First animals ("Protos" = first, "Zoon" = animal)
Protozoans were not seen until the invention of microscope (possibly as early as 1590). In 1676 Leeuwenhock first described microscopic animals and called them animal-cules. Lendermuller (1760-63) gave the name of Infusoria and kept these animals in that group.
In 1809 Lamarck included Protozoans and Rotifers in Infusoria. In 1812 Dutrochet recognized the different organization of Rotifers. Only in 1838 Ehrenberg separated Rotifers from Protozoans. In 1818 Goldfuss gave the name Protozoa. He used this name to represent the lower groups of Cuvier's. In 1845 Von Sieboki recognized the unicellular nature of the protozoans. He gave the definition of protozoa. Perty in 1852 coined the name Ciliata. In 1853 Cohen coined the name Flagellata. In 1879 Leuckart proposed the name Sporozoa. Dobell called protozoans as acellular as their body is not divided into cells. (Hyman also prefered to call them acellular).

The Protozoans are acellular animals without tissues or organs. They exist singly or in colonies. In these colonies all cells are alike. But in "Proterospongia" a colonial flagellate some degree of differentiation exists among the individuals forming the colony.

In some Protozoans all the functions are performed directly by un differential protoplasm. In some Protozoans a functional differentiation is attained. Such differentiations are carried on by organelles. They are also called organoids, or organites.
They are -
  1. Contractile Vacuole" for water regulation in the body.
  2. Photoreceptors" as sensory structures.
  3. Cilia and flagella" for locomotion and food collection.
  4. Skeletal secretions for protection and to give shape.
These organelles resemble different structures of multicellular organisms.
In Protozoans respiratory differentiation is absent. The presence of excretory mechanism is much discussed.
In Protozoans reproduction is by Asexual and Sexual methods. Asexual reproduction is by fission, budding, multiple fission etc.. Sexual reproduction is varied. The syngamy is isogamous. anisogamous and oogamous types. Some organisms show complicated life histories which include alternation of genera tions.
The phylum Protozoa includes Flagellate. Rhizopoda Ciliata and Sporozoa. Formerly Rhizopoda were considered the most primitive protozoa. But they are derived from Flagellata. This view is now universally accepted Flagellates are considered the primitive Protozoans.
The Protozoans are very small in size For example 12 sporozoan parasites (Babasia) can inhabit one Red blood cell. In a single cell several hundreds of protozoans can live. Ex Flagellate parasite " Leishmanial They are very small and are measures in microns The smallest protozoans is "2 to 3 microns and the larger ones may measure upto "250 microns" in length. Porospora gigantea a sporozoan measures 16 m.m. length.
Nearly 50, 000 protozoan species are described and probably many more are to be described.
In Shark 3 pair of internal ears are present. They are organs of equilibrium and hearing. The middle and external ears are absent. The internal ear is a complex structure. It is called the membranous labyrinth. It is present in the auditory capsule of the cranium. It is covered by cartilaginous labyrinth. A space is present between the membranous and cartilaginous labyrinths. It is called peri lymphatic space. It contains peri lymph. It protects and transmits vibrations to the membranous labyrinth. Connective tissue extends from the Cartilaginous to the membranous labyrinth.
The membranous labyrinth has laterally compressed vestibule which is divided into two chambers,
a) The upper narrow utricles,
b) Lower wide sacculus.
The two chambers are connected with each other by a wide passage, the sacculoutricular duct. The sacculus gives off a conical projection from its lower side, called lagena. A narrow canalarisesfrom the dorsal side of the sacculus called ductus endolymphaticus. It pierces through the roof of the cranium and open out by a minute pore. Before it opens out the duct dilates into saccus endolymphaticus. Three semicircular ducts are present. They are called the external, anterior and posterior semicircular canals. . The external canal is horizontal and the other two are vertical in position. All the semicircular anals open into the utriculus at both of their ends. One end of each canal is dilated into oval ampulla. The ampullae of the anterior and the external canals lie close together towards the anterior end of the ear.
The cavity in the membranous labyrinth is filled with a fluid Endo lymph. Six sensory patches are present in the wall of the internal ear.The sensory patches' of the ampullae are called the "cristae". The sensory patches of the utriculus, sacculus and lagena are called maculae acousticae They contain neurosensory and supporting cells. Each neurosensory cell bears a sensory hair at the free end, and passes into a nerve fiber at the other end. The nerve fibres join the auditory nerve. The sensory hair are much longer cristae than in the maculae. They are enclosed in a gelatinous substance. The gelatinous substance is called the cupule. In Maculae the otolithic membrane has in its outer part minute crystalline bodies called otoliths.
They are formed of calcium carbonate.
  1. The cristae detect turning movements of the head.
  2. The maculae in the utriculus and sacculus detect changes in the position of the body at rest and in the forward movement. With a change in the position of the body, the otoliths change their position due to gravity and bring pressure on the underlying sensory hairs and this stimulates the neurosensory cells. The body position is corrected. Thus equilibrium is brought forward.
  3. The maculae & lagena will detect under water sound vibrations to some extent.
Shark two eyes are present on the head. They are located in the orbits and are laterally directed.
The eye is large in size and elliptical body. It is a hollow structure. It is covered by three coats.
1) The fibrous outer coat is thick. It protects the eyeball and maintains its form. It has large posterior scierotic and anterior transparent cornea The sclerotic coat is is hidden in the orbit. A small portion is visible externally, called the white of the eye. The cornea is composed of connective tissue. It is covered by a transparent epithelium called conjunctiva. The cornea permits the light to pass into the eye.
2) The middle coat has three distinct regions.
   a) The choroidis composed of a soft connective tissue with pigment cells and blood vessels
   b) The choroid is adjacent to the retina and contain light reflecting crystals called tapetum lucidium.
   c) At the junction of the sclerotic and the cornea the middle coat, abruptly bends into the cavity of the eye as "iris", it has slit called pupil. Behind the periphery of the iris, the middle coat is thicker, and less pigmented and is called the ciliary body. It has no muscles and plays no role in accommodation.
3) The retina is very delicate and lines the whole of the middle coat, it has three regions.
   a) optic part is in contact with the choroid. It is sensitive to light,
   b) The ciliary and
   c) Iridial parts are non sensory parts.
The point of the retina from where the optic nerve leaves the eye is called the blind spot. This region has no receptors. It is not sensitive the light. The retina contains rods. The cones are absent. The fish is colour blind. Its eye is adapted to "dim light" vision.
A solid transparent crystalline lens, lies just behind the pupil. It is held in place by a gelatinous suspensory ligament. It extends from the lens capsule to the ciliary processes. The lens divides the cavity of the eye into two chambers. The anterior smaller aqueous chamber and the posterior larger vitreous chamber. The aqueous chamber is partly divided into anterior and posterior parts by iris. It has aqueous humour. The vitreous chamber is filled with the vitreous humour. The humours keep the eye inflated and aid in focusing Sight rays on the retina  of shark.
The eye is protected by three lids. The upper end lower lids are folds of skin are immovable. The third eyelid, the nictineting membrane is well developed. It covers the eye fully. It is a growth of the anterior region of the lower eyelid. Lacrimal gland is absent.

Ten pairs of cranial nerves arise from the brain at different locations to supply the anterior region of the body. In addition to them a pair of terminal or pre olfactory nerves (zero nerves) arises from the ventral surface of the cerebrum.

The ten pairs of cranial nerves are furnished below for detail study.