174 ZOOLOGY. 



The mouth is rough with denticles, and has a small, jmmov- 

 able tongue on its floor. The next part of the canal, contain- 

 ing spiracles and gill-slits in its walls, is called the pharynx : 

 we will describe this in the next paragraph. It is followed 

 by a wide c&sophagus passing into a U-shaped stomach, 

 having, like the rabbit's, the spleen on its outer curvature. 

 There is no coiling small intestine, but the short portion 

 receiving the bile-duct and duct of the pancreas is called 

 the duodenum. The liver has large left and right lobes, and 

 a small median lobe in which the gall-bladder is imbedded. 

 The pancreas is compact, as in the frog, and is partially 

 divided into two lobes. The next segment of the intestine 

 is often called the colon, but more probably is the ileum : 

 the colon in the rabbit is the chief region of absorption of 

 water y but fishes excrete no water-vapour as the hot-blooded 

 air-inhabiting rabbit does, and so have little need of a 

 specialized colon. This ileum contains a spiral valve, the 

 shelf of which points steeply forward ; this acts just like 

 a spiral staircase, the chyme having to travel round and 

 round in its passage along the intestine. It is consequently 

 exposed for a longer time and to a larger surface for 

 absorption. In frog and rabbit the same result is attained 

 by lengthening the intestine itself. The final portion of 

 the canal is short and has connected with it a thick 

 cylindrical rectal gland, which has no particular resemblance 

 to the gland of the same name in the rabbit. As in the 

 frog, there is a cloaca. 



7. Respiration. The respiratory apparatus is adapted 

 to water-breathing, the gases concerned being dissolved in 

 the water, and consists of the mouth, pharynx, and gill- 

 slits. The latter bear on their anterior and posterior walls 

 the actual gills highly vascular filamentous outgrowths 

 serving to expose the blood to the oxygen- containing water. 

 There is no palate, narial passage, glottis, larynx, or lungs. 



Seen from the inside (fig. 90), the gill-slits are very large 

 openings, occupying almost the whole of the sides of the 

 pharynx, their external apertures being greatly contracted. 

 It is easy to see, too, that the spiracle is another gill-slit 

 whose ventral portion has been obliterated. The portions 

 of the lateral walls of the pharynx between the slits are 



