718 COMPARATIVE ANATOMY 



ever, develop both types of ribs and even additional ones at various levels 

 of the myosepta. 



The vertebrae are connected with each other by a strand of noto- 

 chordal tissue that perforates all the vertebrae like the string through 

 a chain of beads. 



The fins have ray-like supports of cartilage and the pectoral and 

 pelvic limb-skeletons are supported upon simple horseshoe-shaped pec- 

 toral and pelvic girdles, each composed of a single piece of cartilage. 



AMPHIBIA 



A short cervical and sacral region appear in Amphibia, the cervical 

 becoming longer in the higher forms of vertebrates. The pelvic girdle 

 lies free, in fishes, but in all other vertebrae it is immovably attached to 

 the sacrum. Three fingers develop early in the amphibian foot, although 

 a fourth appears quite late in development (Fig. 421). This fourth finger 

 lies well down on the ulnar side of the hand. Then a rudiment of the 

 fifth (the little finger) appears as a mere bump. The thumb, index, and 

 second finger, therefore, seem to be phylogenetically the oldest digits. 

 This is important in connection with the loss of fingers in other verte- 

 brates, as the last to develop is usually the first to be lost. In the am- 

 phibian we find feet instead of fins. This brings a change in the type 

 of movements in the animal, for with fins, an animal can only paddle 

 backwards and forwards. The muscles are, therefore, decidedly differ- 

 ent and nearly all trace of the segmental arrangement in them is lost. 

 Animals which live on land are relatively heavier than those whicn live 

 in water, so there is need of a much more rigid axial skeleton as well as 

 stronger limb girdles, and limb skeleton. This condition is brought 

 about by a more complete ossification of the parts of the skeleton that 

 bear the most weight. Exoskeleton parts also tend to disappear so that 

 in modern amphibia, the exoskeleton is entirely absent with the excep- 

 tion of the Caecilians, where it is rudimentary. In the Stegocephalians 

 there is a head armor while the exoskeleton is lacking on the rest of the 

 body. The sternum first appears in amphibia. 



REPTILIA 



In reptiles, birds and mammals the cervical region is longer than 

 that of the Amphibia and the trunk region is divided into an anterior 

 thoracic region with long ribs and a more caudal lumbar region with 

 short ribs or with none. 



In all vertebrates rudimentary ribs are usually found on the cervical 

 and sacral vertebrae when these regions are present. 



Fossil remains show that there were many more plates and scutes 

 on the turtles of the past than on those of the present. Both longitudinal 

 and transverse rows of elements have disappeared, the whole system 



