240 FOMPARATIVE MORPHOLOGY OF VERTEBRATES. 



injury (fig. 246, A). In other fishes the posterior margin of the hyoid 

 septum grows back as a broad fold over the clefts behind, thus forming 

 a gill cover or operculum (fig. 246, B, 0), enclosing an extrabran- 

 chial or atrial chamber into which all of the clefts empty and which 

 in turn opens to the exterior by a single slit (od) behind the operculum. 

 This opercular opening is usually broad, but it is reduced to a circular 

 opening on either side in a few teleosts, while in the symbranchii the 

 openings of the two sides are united to a single one in the mid-ventral 

 line. Correlated with this protection of the gills by the operculum is 

 the reduction of the interbranchial septum (fig. 246, B'}, which forms 

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

 gill chamber. 



FIG. 247. Head of Chlamydoselache, after Garman;/, opercular fold. 



Usually the two opercular folds are continuous beneath the pharynx, 

 which points to the beginnings of an operculum in the shark, Chlamy- 

 doselache (fig. 247). In the chimaeroids the operculum is farther 

 developed and is supported by cartilaginous rays. In the teleostomes 

 two parts may be recognized in the operculum, the operculum or gill 

 cover proper, supported by a series of large bones (p. 77), and a more 

 ventral part, the branchiostegal membrane, which is very flexible 

 and has a skeleton of slender (branchiostegal) rays, connected with 

 the hyoid. 



In the sea horses and pipe fishes (lophobranchs) the gills form small rounded 

 tufts. In the labyrinthine fishes there is a complicated bony structure in the bran- 

 chial chamber, covered by a folded membrane which is used in aerial respiration. 

 In the young crossopterygians (Polypterus, Calamoichthys) bipinnate external gills 

 persist for some time. In Amphipnous, just referred to, a sac opening between 

 the hyoid and the first branchial arch is developed on either side of the head. 

 Its walls are very vascular thin vessels being connected with both the branchial 

 arteries and the dorsal aorta. 



The gills are so placed that there can be an almost continuous stream 

 of water over them, thus bringing the oxygen needed by the blood. As 

 a rule, this water is drawn in through the mouth by the enlargement of 

 the oral cavity, and by its contraction is forced out through the clefts. 



