THE RESPIRATORY SYSTEM 



247 



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

 branchial arch laterally towards the skin. 



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 

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



FiG. 228. — 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; 

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

 Vertebrates.") 



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. 228) 



The mechanism of breathing diflfers considerably in fishes which, like 

 the elasmobranchs, have modified the first gill-slits into spiracles, and 

 those which have not. In all fishes, through the action of antagonistic 

 pharyngeal muscles, the cavity of the pharynx is alternately expanded 

 and contracted, so that water is sucked in through the mouth or the 

 spiracles and forced out through the gill-slits. In forms with an opercu- 



