278 FISHES CHAP. 
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). 
t 
B eba 
Fig, 161.—A, Horizontal section through the head of an Elasmobranch; B, similar 
section of a Teleost (diagrammatic). 6.c, Branchial cavity ; 0./, branchial lamellae ; 
c, coelom ; ¢..a, external branchial aperture ; hy.a, hyoid arch ; hy.c, hyo-branchial 
cleft ; /.s, interbranchial septum ; 7, nasal organ ; 0¢s, oesophagus ; op, operculum ; 
p.q palato-quadrate cartilage : Ph, pharynx ; sp, spiracle ; s.ys, spiracular pseudo- 
branch ; 1-5, Ist 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 inter-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 
