256 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



tends some distance beyond the demibranchs, thus differentiating an 

 excurrent canal in the cleft. This prolongation of the septum bends 

 caudally at the outer end, thus protecting the delicate gills from in- 

 jury (fig. 274, A). 



In all other fishes (teleostomes and chimaeroids) the posterior end 

 of the hyoid septum grows back as a broad fold over the clefts behind, 

 thus forming a gill-cover or opercular apparatus (fig. 274, B, 0) which 

 encloses an extrabranchial or atrial chamber into which all the clefts 

 empty and which, in turn, opens by a single slit {00) behind the oper- 

 culum. This opercular opening is usually long in the vertical direc- 

 tion, but it is reduced to a circular opening on either side in a few 

 teleosts, while in the symbranchiates there is a single opening in the 

 mid-ventral line for the two sides of the body. 



Correlated with the protection of the 

 gills by an opercular apparatus is the re- 

 duction of the interbranchial septum 

 {cf. fig. 274, A', B'), so that it forms 



Fig. 275. Fig. 276. 



Fig. 275. — Head of Chlamydoselache, after Garman; /, opercular fold. 

 Fig. 276. — Ventral side of Schilbeodes, after Jordan and Evermann, b, branchiostegal 

 membrane; i, isthmus; o, opening from atrial cavity. 



only a slender bar, from which the demibranchs project far into the 

 atrial chamber. Usually the two opercular folds are continuous 

 beneath the pharynx, which points to the beginnings of an operculum 

 in the oldest living shark, Chlamydoselache (fig. 275). In the 

 chimaeroids, as has just been said, there is a well-developed oper- 

 cular fold which is strengthened by cartilaginous rays arising from 

 the hyoid arch. In the teleosts and ganoids two parts are differenti- 

 ated in the opercular apparatus, the operculum or gill-cover proper, 

 supported by a series of large cartilages or bones (p. 85), and a more 

 ventral part, the branchiostegal membrane (fig. 276) which is very 

 flexible and has a skeleton of slender branchiostegal rays. In these 

 cases the ventral wall of the pharynx is reduced to a slender bar, the 

 so-called isthmus. 



In all cases the gills and accessory structures are such that an 

 almost continuous stream of water is passed over them, thus bringing 



