221 



VERTEBRATE SKELETON 



body when at rest. There is less shifting or torsion in the pelvic 

 fin, but here the fins are so placed that the preaxial border is lateral, 

 the postaxial turned towards the body. 



Except in a few primitive Gnathostomes, the skeleton of the limbs 

 (always laid down in cartilage) becomes differentiated into a support 

 (girdle) enclosed in the trunk, and the skeleton of the free append- 

 age. Thus there are two girdles, pectoral (shoulder) and pelvic, 

 and fore and hind limbs (either pterygia or podia) according to 

 their position. 



The pectoral girdle is always near the heart, be that organ near the gill-clefts 

 or much farther back. Normally the pelvic girdle is near the anus and thus 

 remote from the anterior limbs, but in some Teleosts the pelvic fins may move 

 forwards to a point nearly between the pectoral tins (the old group of 'Thora- 

 cici'), or even in front of them (' Jugulares'). 



Girdles 



The typical girdle (fig. 241) is an inverted arch in the somatic 

 wall of the trunk, extending across the mid-ventral line and upwards 



on either side of the body. The 

 pectoral girdle is frequently con- 

 nected with the axial skeleton (skull 

 or vertebral column) in fishes, but 

 not in Tetrapoda. The pelvic girdle 

 reverses these relations, being free in 

 fishes, while in Tetrapoda, where the 

 hind hmbs bear much of the body 

 weight, the pelvic girdle is attached 

 to one or more sacral vertebras. 



The skeleton of the free append- 

 age is articulated to the corre- 

 sponding side of the girdle, the 

 point of attachment to the shoulder 

 girdle being the glenoid region, 

 that on the pelvis the acetabular 

 region/ These regions serve to separate dorsal and ventral parts in 

 each girdle, a scapular region above the glenoid and a coracoid region 

 below it in the pectoral girdle; while in the pelvic girdle these are an 



1 In fishes the articular surfaces of the girdles are convex or cylindrical; in Tetrapoda 

 are usually excavate and cup-shaped and then are called glenoid fossa and acetabulum 

 respectively. 



Fig. 241. — Pectoral girdle and fin- 

 skeletons of Scylliutn. c, coracoid 

 region; g, glenoid surface; ms, mesop- 

 terygium; ml, metapterygium; p, pro- 

 pterygium; r, radialia; s, scapular 

 region. 



1 



