CUM r AKATIVE ANATOMY 



The dentary, splenial, and angulare are usually found, but very few 

 vertebrates have them all. 



In the hyoid and branchial arches the outside portions are known 

 by the same names as the corresponding cartilages, membrane bones 

 never being found here. 



The method by which the jaws are suspended varies. If the ptery- 

 goquadrate is directly connected with the cranium as in a few elasmo- 

 branchs, the suspension is called amphistylic. If it is held in place by 

 ligaments and the hyomandibular is interposed between the otic capsule 

 and the hinder end of the jaw, it is called hyostylic, while, if the pterygo- 

 quadrate is more or less fused with the cranium, as in all the higher forms 

 beyond the fishes, it is known as autostylic. 



THE APPENDICULAR SKELETON 



There are two types of appendages, namely: median or azygos, 

 which are found in aquatic vertebrates, and the regular paired appen- 

 dages found in all other classes except the cyclostomes. Several theories 

 have been advanced to account for the two types. One of these is that 

 the two types of appendages have no relation to each other, and devel- 

 oped independently, the pelvic and pectoral girdles being supposed to 

 have originated from the gill arches, while the appendage bones have 

 been derived from that portion which normally supports the gills. The 

 other view assumes that two longitudinal folds ran the full length 

 of the body behind the head (Fig. 415), each of these folds being sup- 



Fig. 415. 



Diagrams showing A, the undifferentiated condition 

 of the paired and unpaired fins in the embryo, and B, 

 the manner in which the permanent fins are formed 

 from the continuous folds. AF, anal fin ; An, anus ; BF, 

 pelvic fin ; BrF, pectoral fin ; D, dorsal fin-fold ; FF, 

 dorsal fin ; RF, dorsal fin ; SF, tail-fin ; S, S, lateral folds 

 which unite at S' to form ventral fold. (From Wieders- 

 heim.) 



ported by a series of skeletal rods. The two dorsal and two ventral 

 folds then fused to form the dorsal and ventral fin. The anal opening 

 is, however, on the ventral side. Consequently, the caudal fins had to 

 be formed from the ventral fusion behind the anal opening, while the 

 portion anterior to the anal opening develops into the appendages proper. 



