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The skeletal system or skeleton includes the hard parts of the animal body. All the vertebrates possess the endoskeleton (inside the body).
 
These are the living tissues develop from the interior layers of mesoderm of the body. The term skeleton denotes the endoskeleton in vertebrates. Skeleton plays a very important role in the life of vertebrates. It forms a strong frame work and gives a definite shape to the animal body. It also protects the chief organs of the body such as brain, spinal cord, sense organs, heart and lungs. It provides hard surface for the attachment of muscles which exhibit effective movements. Blood corpuscles are produced from the narrow part of the limb bones.
 
Two main parts present in Endoskeleton. 1. Axial skeleton and  2. Appendicular.
 
Paired appendages articulate with the axial skeleton by means of girdles. The shoulder or pectoral girdle is present in the anterior side of the body to which the pelvic fins or forelimbs are attached. The pectoral girdle is connected to the vertebral column through muscles and ligaments. The pectoral girdle has two equal halves which are known as 'ossa innominata'. Each of innominatum possess the dorsal bone - Scapula, the ventral bone ‘Coracoid' and antero - ventral bone - 'clavicle'. At the junction of scapule and coracoid, a depression or concavity is formed. It is known as glenoid cavity into which head of lumerus of the forelimb articulates and forms a ball and socket joint.
 
pectoral girdle frog thumb15
 
In the different vertebrates the same bones are present in pectoral girdle with some modifications.
 
Shark (Scoliodon) - Pectoral Girdle Frog (Rana) - Pectoral Girdle
1. Pectoral girdle is present in the body wall muscles below the last branchial arch. 1. Pectoral girdle is present in the anterior part ot the trunk.
2. It is not connected to the axial skeleton and formed with cartilage tissues only. 2. It is connected to the axial skeleton (vertebral column) dorsally with muscles & ligaments and formed mostly with bone tissues.
3. Pectoral girdle has two halves. 3. Same as in shark.
4. The osinnominatum has a thin flat, ventral coracoid and a thick rod-like dorsal scapula along with clavicle bones. This type of girdle is known as scapulo-coracoid bar. Supra scapula is absent. 4. The os innominatum has the dorsal scapular part and ventral coracoid part. Stouter scapula on the lateral side and the supra scapula formed of calcified cartilage which is a* thin, broad present on the dorsal side. The clavicle membrane bone is present antero-ventral to the pectoral girdle.
5. The coracoid bone is poorly developed and present on the ventral side. It is not differentiated into pre, and epi coracoids. 5. The coracoid part possess, proper stout uralid, infront of it calcified cartilaginous pre coracoid along with another strip cartilaginous epicoracoid bones. The two epicoracoids are closely associated with each other in the mid ventral line.
6. Glenoid cavity is absent. 6. At the junction of scapular & coracoid bones on the posterior side, the depression of glenoid cavity is present.
7. Sternum is absent. 7. The sternum lies in the mid ventral line of the chest part. It has two portions. The anterior presternum attached to the anterior part of the girdle. It possess a X - shaped omosternumand at its distal end a small carlilaginous episternum. The posterior purt sternum is present behind the girdle. It consists of a stout bone mesosternum and flat plate xiphisternum at the distal end.
8. The basal cartilages of the pec-toral fin are articulating with the pectoral girdle. 8. The forelimb bones are articulating with pectoral girdle.
9. Coracoid bones bear on its part a few formina for blood vessels. 9. Coracoid fenestrae are present.
10. It helps only for the movements of the pectoral fins and also protects the heart. 10. It not only helps the movements of the fore limbs but also protects the heart and lungs.

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