484 TELEOSTEI [CH. 



the stomodaeum. The circumstance that teeth are borne by the 

 lining of the pharynx covering the pharyngeal bones is probably to 

 be attributed to an ingrowth of ectoderm through the gill-slits. 



Turning now to the internal anatomy of the Teleostei,'we shall 

 emphasise the points in which structure of a typical Teleostean 

 differs from that of a typical Cartilaginous fish described on pp. 460 

 466. We shall deal first with the alimentary canal. 



The gill-slits are separated from each other by narrow septa, 

 so that they are mere slits, not sacs as in the Chondrichthyes. The 

 spiracle has disappeared. The gills themselves are long triangular 

 filaments set in longitudinal rows on each side of each gill-septum, 

 or gill- arch as we may term it, and they project outwards into the 

 cavity between the gill-cover or operculum and the gill-arches. On 

 the anterior wall of the slit between the hyoid arch and the first 

 branchial arch there are no gill-filaments but usually a small round 

 red body richly supplied with blood-vessels, and termed the pseudo- 

 branch. This may be a vestige of the gill belonging to the front 

 wall of the first gill-cleft. Some think it represents the vestigial 

 gill of the lost spiracle. 



When a Teleostean fish opens its mouth to breathe the flexible 

 part of the opercular flap supported by the branchiostegal rays is 

 pressed against the body and prevents the exit of water through the 

 gill-slits. Then when the pharynx is constricted in order to drive 

 the water through the slits, the operculum is lifted and at the same 

 time two curtain-like folds of membrane borne by the maxillae swing 

 out and meet, and prevent the escape of water through the mouth. 

 If the flexible part of the operculum be called the posterior 

 breathing valve, then the curtains may be regarded as anterior 

 breathing valves. At the pyloric end of the stomach there are 

 usually a large number of blind outgrowths termed pyloric caeca, 

 and, intermixed with them and difficult to detect, a number of 

 delicate tubules which represent the pancreas. Above the stomach 

 lies the air-bladder, which may or may not open in the mid-dorsal 

 line by a duct into the oesophagus or the beginning of the stomach. 

 The intestine is long and thrown into several loops, and is devoid 

 of a spiral valve. - It opens behind by an anus which is distinct 

 from the openings of the kidneys and genital organs. 



In the male the kidney has no connection with the testis, so 

 that the kidney cannot be divided into regions but constitutes one 

 uniform elongated organ which opens into an elongated archinepliric 

 duct. The two archinepliric ducts unite into a single median duct, 



