106 



COMPARATIVE ANATOMY 



previously put forward the view that the arches and fins correspond to meta- 

 morphosed gill-arches and rays : he supposed that one ray came to exceed 

 the others in size, and that the others then gradually became attached to it 

 instead of to the arch, the result being a biserial form of fin (" archiptery- 

 ") which is most nearly retained in Ceratodus (Fig. 101 and p. 124). 



Pectoral Arch. 



Fishes and Dipnoans. Paired fins and arches are wanting 

 in the Cyclostomi. In the Elasmobranchii and Holocephali the 

 pectoral arch consists of a comparatively simple cartilaginous bar 



FIG. 83. PECTORAL ARCH AND FIN OF Heptanchu*. 



SB, SB 1 , pectoral arch, with a nerve aperture at NL ; Pr, Ms, Mt, the three 

 basal elements of the fin pro-, meso-, and metapterygium ; Ra, cartilaginous 

 fin-rays ; a, b, the main fin-ray, lying in the axis of the metapterygium ; 

 t, single ray on the other side of the axis (indication of a biserial type) ; FS, 

 horny rays, cut through. 



the two halves of which are united ventrally by cartilage or fibrous 

 tissue (Fig. 83), and in embryos of Ganoids and Teleosts it has 

 at first a similar structure. 



Later, however, in both the last-named groups, a row of bony 

 structures arises in the perichondrium in this region ; so that a 

 secondary or bony pectoral arch may be distinguished from a 

 primary or cartilaginous one, the latter becoming less marked in 

 proportion to the development of the former (Fig. 84). 



The free extremity, or fin, is always connected with the hinder 



