TELEOSTEAN FISIM.S 



355 



embryo, never becomes functional or even perforate so far as is known 

 in Teleosts. 



The breathing of the fish is carried out by characteristic movements 

 which ensure the respiratory lamellae being bathed by a fresh stream of 

 water. The pharynx is dilated by the action of muscles attached to the 

 skeleton of the branchial arches and water is thus drawn in through the 

 widely open mouth, the oper- 

 culum at the same time being 

 sucked in against the body-wall 

 so as to prevent any indraught 

 through the opercular opening. 

 After the pharynx is filled in this 

 way the branchial musculature 

 brings about a contraction which 

 tends to force the water out again. 

 The mouth is closed, the closure 

 being completed by valvular flaps 

 within the lips. Thus no water 

 passes forwards through the 

 mouth opening. It all passes 

 backwards, the operculum open- 

 ing so as to allow a free exit. 

 The rhythmic repetition of these 

 respiratory movements causes a 

 continual pumping of water in at 

 the mouth, over the surface of 

 the lamellae and out by the oper- 

 cular opening. 



In connexion with the res- 

 piratory region of the alimentary 



Canal there exists in the typical E). 



Teleost a characteristic organ 

 known as the air-bladder or swim- 

 bladder. This arises during development as a pocket-like outgrowth 

 of the wall of the alimentary canal, as a rule near the mid-dorsal 

 line though sometimes well to one side or the other. The outgrowth 

 reaches a large size, extending tailwards immediately dorsal to, but 

 outside, the splanchnocoele (see Fig. 154, D, p. 367). Commonly an 

 outgrowth of the air-bladder wall bulges forwards towards the head, 

 so that the original connexion with the alimentary canal, now narrow 

 and tubular (pneumatic duct), is attached not to the headward end of the 



FIG. 149. 



Illustrating the relations of the respiratory 

 lamellae and gill-septa in an Elasmobranch (A), a 

 modified Elasmobranch Chimaera (B), a Sturgeon 

 Acipenser (C), and two different Teleosts (D and 

 (From Boas, The Cambridge Natural History, 



