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FISHES 



series of inter-branchial septa, each of which consists of the lining 

 membrane of two contiguous clefts and a median fibrous sheet ; 

 it is further strengthened on its pharyngeal margin by a 

 branchial arch, and more externally by the fringe of cartilaginous 

 rods (branchial rays) with which the outer convex edge of each 

 arch is provided. The anterior and posterior walls of each 

 septum are produced into a number of outwardly -radiating 

 vascular plates or folds (branchial lamellae or " gills "), which by 

 their free edges project into the cavity of the cleft (Fig. 161, A). 



e.b.a, 



FIG. 161. A, Horizontal section through the head of an Elasmobranch ; B, similar 

 section of a Teleost (diagrammatic), b.c, Branchial cavity ; b.l, branchial lamellae ; 

 c, coelom ; c.b.a, external branchial aperture ; hy.a, hyoid arch ; hy.c, hyo-branchial 

 cleft ; l.s, interbranchial septum ; n, nasal organ ; oes, oesophagus ; op, opereulum : 

 p.q, palato-quadrate cartilage : P/i, pharynx ; sp, spiracle ; s.ps, spiracular pseudo- 

 branch ; 1-5, 1st to 5th branchial arches. (From Boas, slightly altered.) 



Although slightly free at their outer extremities, the lamellae do 

 not extend so far as the external margin of the septum to which 

 they are attached (Fig. 164, B). Each series of lamellae is termed 

 a " hemibranch," and, from what has been said, it is obvious that 

 each iriter-branchial septum and its supporting branchial arch carry 

 two hemibranchs, an anterior and a posterior, the two forming a 

 complete biserial gill or " holobranch." The hyoid arch, how- 

 ever, has only a single hemibranch, viz. that pertaining to the 

 anterior wall of the hyo-branchial cleft, and as the fifth or last 

 cleft has a hemibranch only on its anterior wall, the fifth arch is 



