128 FISHES. 



coecal diverticulum of the stomachic dilatation. A mesenterium 

 i^ absent. 



In the Gyclostomi the intestinal tract is likewise straight, 

 and without clearly defined divisions ; however, in Peiromyzon 

 the oesophagus shows numerous longitudinal folds, and the 

 intestine proper is provided with a single longitudinal fold. 

 A mesentery, which is present in the Myxinoids, is represented 

 by a short median fold only, by means of which the hind- 

 most part of the intestine is fixed. 



The Palceichthyes show differences in the structure of their 

 intestinal tract as considerable as are found among the 

 Teleostei, but they have that in common that the absorbent 

 surface of their intestine is enlarged by the development of a 

 spiral valve, evidence of the presence of which in extinct 

 Palseichthyes is still preserved in the fossilised faeces or 

 coprolifhs, so abundant in some of the older strata. 



In Chondropterygians (Fig. 55) the stomach is divided into 

 a cardiac and pyloric portion, the former frequently terminat- 

 ing ia a blind sac, and the latter varying in length. The 

 pyloric portion is bent at its origin and end, and separated from 

 the short duodenum (called Bursa entiana in these fishes) by 

 a valve ; the ductus hepaticus and pancreaticus enter the 

 duodenum. This is succeeded by the straight intestine pro- 

 vided with the spiral valve, the coils of which may be either 

 longitudinal and wound vertically about the axis of the 

 intestine, as in Carcharias, Galeocerdo, Thalassorhinus, and 

 Zygmna, or they may be transverse to that axis, as in the 

 other genera. The number of gyrations in the latter case 

 varies : there may be as many as forty. The short rectum 

 passes into a cloaca, which contains also the orifices of the 

 urogenital ducts. Only the commencement and end of the 

 intestinal tract are fixed by mesenterial folds. 



In the Holocephali and Dipnoi, the intestinal tract is short, 

 straight, and wide, without stomachic dilatation, a pyloric 



