XIII 



PHYLUM CHORDATA 



75 



branchial arches undergo more or less complete atrophy, the whole 

 gill- bearing apparatus becoming reduced to a small hyoid bone 

 serving for the support of the tongue. 



The skeleton of the median fins is formed of a single row of 

 cartilaginous rays or pterygiophores (Fig. 715, C and D,/.r), lying in 

 the median plane and more numerous than the vertebrae. They 

 may ossify, and may be supplemented by dermal fin-rays, formed 

 either of bone or of a horn-like material, and developed in the 



Bas 



FIG. 721. Diagram of three stages in the development of the pelvic fins. In A the anterior 

 pterygiophores on the right side (Rad), have united to form a basal cartilage (Bas.); in B the 

 basalia (Baft.) are fully formed and are uniting at * to form the pelvic girdle ; in C the pelvic 

 girdle (G) is- fully constituted, and at t has segmented from the basale on the right side. 

 Cl. cloacal aperture. 



derm along the free edge of the fin. The latter are clearly 

 exoskeletal structures. 



As already mentioned, the paired fins are probably to be looked 

 upon as the detached and enlarged anterior and posterior portions 

 of a continuous lateral fin the intermediate portion of which has 

 disappeared. Both pectoral and pelvic fins are supported by 

 pterygiophores or radialia (Fig. 721, Had), the basal or proximal 

 ends of which are articulated with stout cartilages, often replaced 



