140 



COMPARATIVE ANATOMY 



Pectoral Arch. 



Fishes. Paired fins and arches are wanting in Cyclostomes. 

 In Elasmobranchs the pectoral arch consists of a comparatively 

 simple cartilaginous bar (Fig. 99), situated just behind the 

 branchial apparatus, the two halves of which are united ventrally 

 by cartilage or fibrous tissue, 1 and in embryos of Teleostomes it 

 has at first a similar structure. Later, however, in the last-named 



FIG. 99. PECTORAL ARCH AND Fix OF Heptanckus. 



n, l>, the main fin-ray, lying in the axis of the metapterygium (3ft) ; t, single ray on 

 the other side of the axis (indication of a biserial type) ; F8, horny rays, 

 cut through ; Pr, Ms, Mf, the three basal elements of the fin (pro-, meso-, 

 and metapterygium) ; Ra, fin-rays ; SB, SB 1 , pectoral arch, with a nerve 

 aperture at NL. 



Order, bony structures originating from the integument are 

 developed 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. 100). 



In all Fishes the free extremity, or fin, is connected with the 

 hinder and outer circumference of the (primary) arch, convex 



1 In Heptanchus there is a small ventral element which has been compared 

 to a "sternum." 



