PHYLUM ARTHROPODA: CHARACTERISTICS AND CLASSIFICATION
GENERAL CHARACTERS OF PHYLUM ARTHROPODA
Aristotle described crabs and some decapods. He grouped them as malacostraca in insecta.
The name Arthropod means jointed legs. These animals are meteorically segmented, coelomate and triploblastic.
It is the largest phylum of Animal kingdom.
- These animals are multicellular triploblastic, bilaterally symmetrical animals. Some of the anterior segments show cephalization forming a distinct cephalic region.
- Body is segmented. Segments are limited. Each segment bears a pair of appendages. They are jointed.
- The body is covered by exoskeleton. It is made by chitin. The exoskeleton project inside. The projections are useful for muscle attachment.
- The exoskeleton is cast off periodically and new one is secreted. It is called ecdysis.
- Tubular dorsal heart is present.
- True coelom is greatly reduced. It is represented only by the cavities of the reproductive and excretory organs.
- The blood is colorless.
- Circulatory system is open type or lacunar type.
- The nervous system shows nerve ring and double ventral nerve cord.
- Nephridia are absent. Excretion is by malphighian tubules, green glands and coxal glands.
- Antennules are present in some arthropods.
- Striated muscles are present segmentary.
- Compound eyes are present.
- Cilia and flagella are entirely absent.
- Respiration is by gills, or by trachea, or by book kings, or by gill books etc.
- Sexes are separate, sexual dimorphism is seen. Gonads and their ducts are paired.
- Development is direct or indirect. The life history may include a number of larval forms.
CLASSIFICATION OF ARTHROPODA
Phylum Arthropoda is divided into seven sub phyla.
It includes peripatus. It shows both Annelida and Arthropoda characters. It is a terrestrial form. It is a primitive worm like organism. It shows pairs of eyes, antennae and jaws. It shows tracheal mode of respiration. It has ladder like nervous system. It is connecting link between annelid and arthropoda. Ex: Peripatus.
II. SUB PHYLUM: TARDIGRADA:
These are soft skinned animals of minute size. They are 1 mm in size. They live in fresh water or sea water. Ex: Macrobiotus.
III. SUB PHYLUM: PENTASTOMIDA:
A group of worm like parasites .They do not show circulatory, respiratory or excretory organs. Ex: Uguatula: (Parasite in nasal sinules of dog)
IV. SUB PHYLUM: TRILOBITOMORPHA:
All are extinct. Trilobites are preserved as fossils.
V. SUB PHYLUM: CHELICERATA:
Their body is divisible into anterior prosoma fCephalothorax), and posterior opisthosoma (Abdomen). Prosoma shows 6 pairs of appendages. First pair is chehcerae. Hence they are chelicerate. Antennae and true jaws are absent. Respiration is by gills, tracheae or book kings. Sexes are separate. It is classified into two classes.
1. Class: Merostomata:
Marine arthropods. They show gill respiration. They donot show antennae. This is divided in to two subclasses.
a) Sub Class: Xiphosura: Ex: Limuhis (King crab)
It shows gill books. It has a horse - shoe shaped carapace, and hence it is called horse shoe crab.
b) Sub Class: Eurypterida: They are eatinct.
2. Class: Arachnida:
They are mainly terrestrial. Simple eyes are present. Antennae and jaws are absent. The body is divided into Prosoma; Mesosoma and Metasoma. It is divided into the following orders.
a) Order: Scorpionida: Elongated body. Prosoma is short and unsegmented. Chelicerae are small and 3 segmented. Ex: Palamnaeus, Buthus. !
b) Order: Pseudoscorpionida: SmaD and scorpion like arachnids. Ex: Chelifer.
c) Order: Opiliones (or) Phalangida: These are called harvest spiders. Body is short and oval. Ex: Phalangium.
d) Order: Acarina: It includes mites and ticks. Body is small and oval. Prosoma and opisthosoma are not distinct. Ex: Argas (Bird tick)
e) Order: Araneae: This is the largest order. It includes spiders. Respiratory organs are book lungs. Ex: Agelena.
f) Order: Solifugae: These are sun spiders. They live in dry place. Opisthosoma is of 10 or 11 segments. Telson is absent. Ex: Galeodes.
g) Order: Thelyphonida: Whip scorpions. They measure 18 to 65 mm in size. Ex: Mastigoproctus.
h) Order: Schizomida: These are small and nocturnal. Prosoma is unsegmented. Ex: Thelyphonus.
i) Order: Phrynichida (or) Amblypygi: These are scorpion spiders. Prosoma is unsegmented. Opisthosoma is 12 segmented. Terminal flagellum is absent. Pedipalps are heavy. Ex: Charinus.
j) Order: Ricinulei (or) Podogona: Tick like and small arachnids. Ex: Ricinoides.
VI. SUB PHYLUM: PYCNOGONIDA:
These are small, marine and spider like arthropods. Their body shows one cephalic somite, 3 or 4 trunk somites and a vestgeal abdomen. Respiratory or excretory organs are absent. Mouth is present on a long proboscis. Ex: Nymphon, Pycnogonum (Sea spider)
VII. SUB PHYLUM: MANDIBULATA:
These are terrestrial, fresh waler or marine animals. Their body shows two or three parts. Antennae are 1or 2 pairs. Mandibles are 1 pair. Maxillae are 1 or 2 pairs. Walking legs are 3 or more pairs. Respiration is by gills or tracheae. Excretion by Malphighian tubules or green glands. Sexes are separate. This is divided into 3 classes.
Class 1: Crustacea:
These are mostly aquatic and gill breathers. Their exoskeleton is hard, and impregnated with lime salts. Head shows 2 pairs of antennae and 3 pairs of jaws. Respiration is by gills or body surface. Excretion is by antennal glands.
There are more than 30,000 known species of crustaceans. This is divided into 8 sub-classes.
Sub-Class 1: Cephalocarida: Most primitive, marine and bottom dwelling arthropods. Body is small, slender and divided into a horse shoe-shaped head and a trunk of 18 segments. Eyes and carapace are absent. Larva is a metanauplius. Ex: Hutchinsoniella.
Sub-Class 2 : Branchiopoda: These are small, primitive, free-living. These are mostly fresh water crustaceans. Antennules are the second maxillae are either reduced or absent. Mandibles are with or without a vestigial palp. Larva is nauplius. Parthenogenesis is very common. The sub-class is divided into 4 orders.
a) Order: Anostraca (Fairy shrimps): Ex: Branchipus, Artemia.
b) Order: Notgstraca (Tadpole shrimps): Ex: Trijxs arid Lepidurus.
c) Order: Conchostraca (Calm shrimps): Ex: Lynceus, Lepteatheria.
d) Order: Cladocera (Water fleas): Ex: Daphrila
Sub-Class 3: Ostracoda (Seed shrimps): These are small, free-living fresh water or marine arthropods. Body is laterally compressed and poorly segmented. Body is entirely enclosed within a bivalved carapace with an adductor muscle. Larva is napulius. Parthenogenesis is quite common. The sub-class is divided into 4 orders.
a) Order: Myodocopa: Ex: Cypridina.
b) Order: Cladocopa: Ex: Polycope.
c) Order: Podocopa: Ex: Cypris, Drawinula.
d) Order: Platycopa: Ex: Cytherelia.
Sub-Class 4: Mystacocarida: These are primitive and marine. Their body is microscopic and elongated. Thorax shows 4 segments, each with a pair of simple and reduced appendages. Abdomen is limbless. Ex: Derocheilocarus.
Sub-Class 5: Copepoda: These are free swimming and fresh water or marine arthropods. Their body is small, elongated and composed of a head, thorax and abdomen. Carapace is reduced or absent. Larva is nauplius. Ex: Cyclops, Calanus.
Sub-Class 6: Branchiura: These are temporary ectoparasites of fresh water and marine fishes and some amphibians. Ex: Argulus (carp lice).
Sub-Class 7: Cirripedia (Barnacles): These are marine, sessile, parasitic or fixed to rocks, boats etc. Body is relatively large and imperfectly segmented. Animal is enclosed in a carapace made of two folds of skin called themantle, which is supported by calcareous plates. Typically 6 pairs of biramous thoracic appendages are present. Abdomen is rudimentary. The sub-class is divided into 5 orders.
a) Order: Thoracica: Ex: Lepas, Balanus.
b) Order: Acrothoracia: Ex: Trypetesa.
c) Order: Ascothoracica: Ex: Synagoga, Dendrogaster
d) Order: Apoda: Ex: Proteolepas.
e) Order: Rhizocephala: Ex: Sacculina, Peltogaster.
Sub-Class 8: Malacostraca: These are large crustaceans marine and fresh water arthropods. Body is segmented, with head of 6 segments, a thorax ot 8 segments and an abdomen of 6 segments. Carapace may be present, vestigial or absent. Typically 19 pairs of appendages are seen. The sub-class^ divided into 2 series.
i) Series: Leptostraca: These are exclusively marine and primitive malacostracan with 21 body segments.
Order: Nebaliacea: Ex: Nebalia.
ii) Series: Eumalacostraca:
These are marine, fresh water or terrestrial malacostracans with 20 body segments.
Super-Order 1: Syncarida: It includes two orders.
- Order: Ahapsidacea: Fresh water forms. Ex: Anapsides.
- Order: Bathynellacea: They are small. Pleopods are reduced. Ex: Bathynella.
Super-Order 2 : Peracarida: It is divided into 5 orders.
- Order: Mysidacea: Many are marine and few are freshwater form. Ex: Mysis
- Order: Cumacea: They are smaU and minute. They are seen in deep littoral parts of the sea. Ex: Diastyles.
- Order: Tanaidacea: They are minute marine arthropods. Ex: Tanais.
- Order: Jsopoda: Body is dorsi -ventrally flat. Carapace is absent. Ex: Oniscus, Ligia.
- Order: Amphipoda (Sand hoppers): Body is elongated and laterally compressed. Ex: Gammarus.
Super-Order 3: Hopolocarida: It has a single order.
Order: Stomatopoda (Mantis shrimps): These are marine arthropods. Ex: Squilla, Pseudosquilla.
Super-Order 4: Eucarida: It includes two orders.
- Order: Euphausiacea: They are marine. They form the food of whales. Ex: Euphausia (Krill).
- Order: Decapoda: These are marine fresh water or terrestrial forms. Ex: Penaeus, Hippa.
Class 2: Myrinpoda:
These are terrestrial and air-breathing arthropods. Their body is made of many similar segments. Antennae are 1 pair. Jaws are 3 pairs. Legs are more than 11 pairs. It is divided into 4 groups. Which are not closely related. Now a days these 4 groups of moriapoda are regarded as separate classes.
Group I or Class Diplopoda (millipedes): These are commonly called millipedes. The body is long and cylindrical with distinct head and trunk, which has 11 to 100 or more segments according to the species. The name millipede means 'thousand leggers'. Two pairs of legs are present on each segment.
Sub-Class 1: Pselaphognatha: Ex: Polyxenus.
Sub-Class 2: Chilognatha:
- Order Oniscomorpha: Ex: Glomeris.
- Order Limacomorpha: Ex: Glomeridesmus.
- Order Colobognatha: Ex: Platydesmus.
- Order Nematomorpha: Ex: Polymicrodon.
- Order Polydesmoidea: Ex: Polydesmus.
- Order Juliformia: Ex: Julus.
Group-II or Class Chilopoda (Centipedes): The Chilopoda are commonly called centipedes. The body is slender elongated, and composed of 15 to 180 segments in different species. Chilopoda means "margin foot". Each trunks segment bears a pair of small 7 jointed legs. The first pair of trunk appendages or maxiilipedes form the large prehensile poison claws. It is divided into 4 orders.
- Order Scutigeromorpha: Ex: Scutigera.
- Order Lithobimorpha: Ex: Lithobius.
- Order Geophilomorpha: Ex: Geophilus.
- Order Scolopendromorpha: Ex: Scolopendra.
Group III or Class Pauropoda: It is a small class, comprising of 60 to 70 species. They inhabit moist soil. The body is small. They have distinct conical head. Trunk segments are eleven a telson is present. Ex: Pauropus, Brachypauropus.
Group IV or Class Symphyla: Symphyla is a small class of about 120 known species. The body is small white, elongated, centipede like, composed of 15 to 22 similar segments, 12 of which usually bear legs. Ex: Scutigerella, Hansaniella.
Class 3: Insecta:
These are terrestrial and aquatic arthropods. Respiration is by trachae. Body cavity is haemocoel. Heart is dorsal and tubular. Body is divided into head, thorax and abdomen. Antennae are 1 pair, jaws are 3 pairs, walking legs are 3 pairs, wings are 1 or 2 pairs.
Order 3: Isoptera (equal wings): Ex: Termites or White ants
Order 4: Plecoptera (fold wings): Ex: Stcneflies.
Order 5: Embioptera (lively wings): Ex: Web-spinners (Gligotoma)
Order 6: Psocoptera: Ex: Book lice.
Order 7: Anoplura: Ex: Pediculus (body louse).
Order 8: Mallophaga: Ex: Bird-lice.
Order 9: Thysanoptera: Ex: Thrips (Heliothrips).
Order 10: Ephemeroptera: Ex: Mayflies. (Ephemera).
Order 11: Odonata: Ex: Dragon flies and Damsel flies.
Order 12: Hemiptera (half wings): Ex: Belostoma (water bug), Bedbug (Cimex).
Division II: Endopterygota (Holometabola): Wings develop internally. Metamorphosis is complete including pupal stage.
Order 1: Neuroptera (nerve wings): Ex: Alder flies, Ant lions.
Order 2: Mecoptera: Ex : Scorpion flies (Panorpa).
Order 3: Trichoptera (hair wings): Ex: Caddis flies.
Order 4: Hymenoptera (membrane wings): Ex: Ants, Bees, Wasps.
Order 5: Diptera (two wings): Ex: Goats, Mosquitoes, House flies.
Order 6: Lepidoptera (scale wings): Ex: Butterflies and Moths.
Order 7: Strepsiptera (turning wings): Ex: Stylopids.
Order 8: Coleoptera: (sheath wings): Ex: Beetles.
Order 9: Siphonoptera: Ex: Reas, Pulex.
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