DIGESTIVE SYSTEM OF FISHES. 241 



ol" the uterus or urinary bladder ; the Lepidosiren lias the peculiarly 

 iciitliyic arrangement of the anal, genital, and urinal outlets.* 



In the Dermopteri the intestinal canal is pretty closely attached to 

 the back of the abdomen, though the primitively continuous mesen- 

 teric fold becomes reduced in the Lampreys to filamentary processes 

 accompanying the mesenteric vessels. Rathke has observed a similar 

 metamorphosis of the mesentery in the Syngnathi and Cyprinidee 

 into detached membranous bands. The mesentery is entire in the 

 Lepidosiren, the Plagiostomes, and many other fishes : it is usually 

 single and continuous from the stomach to the end of the intestine : 

 there are two parallel mesogastries in the Eel, and a kind of omental 

 accumulation of adipose matter is sometimes found along the ventral 

 surface of the intestines: a second mesentery is continued from this 

 part of the intestine to the ventral parietes of the abdomen in the 

 Murasna. 



The position of the cloacal outlet varies much in fishes : in some of 

 the jugular species it follows the ventral fins to the region of the throat ; 

 and, in the apodal Gymnotus, it is placed so far forwards as to remind 

 us of the position of the excretory outlet in the Cephalopods. It is 

 beneath the pectorals in the AmbJijopsis spelceus : but the more normal 

 posterior position of the vent obtains in most abdominal and all car- 

 tilaginous fishes. 



Petrified fseces or ' coprolites ' give some insight into the structure 

 of the intestinal canal in extinct species of fishes : some that have 

 been found in the skeleton of the abdomen of the great Macropoma 

 of the Kentish Chalk, and detached coprolites associated with the 

 scales and bones of the more ancient Megalichthys, indicate by their 

 exterior spiral grooves that these ancient Ganoids, like their modern 

 representative, the Polyptervs, possessed the spiral valve. 



The liver makes its first appearance in the lowest vertebrated, as 

 in the lowest articulated species, under the form of a simple cajcal 

 production from the common alimentary canal : commencing in the 

 Lancelot {Jig. 46. hd), a little beyond the orifice pi/, the hepatic 

 caecum (l) extends forAvards by the side of the ciliated respiratory 

 sac, which appears to be the homologue of the long oesophagus with 

 the attached marsipo-branchial organs of the Lampi'eys, but which 

 some may view as representing the stomach of higher fishes. As 

 the true digestive function, however, cannot be supposed to begin 

 until the food has entered the canal ii, the place of communication 

 of the rudimcntal liver corresponds in the Lancelet with tliat in the 



* xxxni. pi. 25. fig. 'I, w, II, o, I. The Hranchiostoma ofFirs no cxcei)tion to this 

 rule ; the opening, by which the ova and semen are expelled, is a common peritoneal 

 outlet. 



VOL. II. B 



