330 



COMPARATIVE ANATOMY 



The gills of elasmobranchs may be taken as typical. They are modifica- 

 tions of the branchial bars or arches which alternate with the gill-sHts 

 and serve to keep them open. Each branchial arch consists of an inter- 

 branchial septum of connective tissue which is covered on the surface of 

 the body by skin, and which includes near the pharyngeal lining a car- 

 tilaginous arch as a support. Within the septum are branches of the 

 dorsal and ventral aortae which supply the gills with blood. The septa 

 are further supported by skeletal gill-rays extending from skeletal bran- 

 chial arch laterally towards the skin. 



Fig. 274. — Diagram of gill clefts in (A) elasmobranchs and (B) teleosts. A' and 

 B', a single gill of each, a, artery; ba, branchial arch; br, branchial ray; d, demibranchs ; 

 gc, atrial chamber; gr, gill raker; o, operculum; oe, esophagus; 00, opercular opening; 

 s, spiracle, in A', septum; v, veins. (From Kingsley's "Comparative Anatomy of 

 Vertebrates.") 



Each interbranchial septum bears on each surface a half-gill or hemi- 

 branch, which together constitute a holobranch. Each hemibranch is a 

 mucous membrane folded into minute parallel lamellae or branchial 

 filaments, each of which has parallel secondary folds containing a capillary 

 network. Between the capillaries and separating them are pilaster cells 

 pecuHar to the gill filaments. In the ganoids and teleosts the inter- 

 branchial septum becomes reduced and tends to disappear, leaving only 

 the portion containing the skeletal arch and branchial blood vessels. In 

 these forms, the gill slits do not open separately to the exterior as in 

 elasmobranchs but are covered by an operculum formed by the backward 

 growth of the septum of the hyoid arch. (Fig. 274) 



