156 STRUCTURE OF THE VERTEBRATES 



E. Origin of the Appendages 



The pectoral and pelvic girdles together form the appendicu- 

 lar skeleton. With the girdle proper, which is the internal bracing 

 structure, is included the appendage — either fin, leg, arm, or 

 wing. 



There are several theories to account for the origin of the 

 paired fins from the finless, jawless ancestors of the fish, but 

 the most acceptable to most anatomists is the fin fold theory. 

 This theory requires an ancestral type with a continuous fin 

 along either side of the body, beginning posterior to the gills 

 and extending posteriorly and ventrally to a point posterior to 

 the anus. At this point they fuse and continue to the end of the 

 animal as the ventral and caudal fins. This animal is supposed 

 to have developed metameric cartilage supports to give strength 

 and rigidity to the long fin fold. Then, during evolution, the 

 middle and most posterior regions of the paired fins disappeared, 

 leaving wide pectoral and pelvic fins. In similar manner the 

 continuous dorsal fin became reduced to two dorsal fins, and the 

 upper portion of the caudal. The evidence from both palaeon- 

 tology and embryolog}" supports this theory. 



The most ancient and primitive sharks are known only as 

 fossils. Of these Cladoselache is the type form. Its notochord 

 was continuous and its vertebrae only slightly better developed 

 than those of the cyclostomes. Its most interesting feature was 

 the appendages, the pelvic fin being more primitive than the 

 pectoral. Each fin had a wide base and was half-moon shaped. 

 The pelvic was supported by metameric, disconnected cartilages; 

 while the pectoral had similar cartilaginous rays and a longi- 

 tudinal series of small cartilages at their base. Eventually this 

 basal series shortened to form the three basal cartilages found 

 in the living sharks. 



The embryological evidence is equally clear. In the elasmo- 

 branch embryo the first sign of the appendages is a double row 

 of limb buds in the pectoral and pelvic regions, (page 158). Each 

 limb arises from six or more segments. As these muscle buds 

 grow each carries a spinal nerve with it, the entire structure 

 extending distally as a wide, rounded embryonic limb. As the 

 limb takes shape the base constricts and soon the definitive fin 



