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 (00) 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. 
Fic. 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 chimzroids 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. 
