424 ANATOMY OF VERTEBRATES. 



it is to be remarked that the intestinal canal is shortest, and 

 the spiral valve most complex and extensive, in the Sharks. 

 In both these and the Rays, the valve subsides at a short distance 

 from the anus ; and into the back part of this terminal portion of 

 the rectum an elongated crccal process with a glandular inner 

 surface opens fig. 352, i. The anus itself communicates with the 

 fore part of a large cloacal cavity in the Plagiostomes. In other 

 Fishes, where it opens distinctly upon or near the external surface, 

 it is anterior in position to the orifices of oviducts, or sperm- 

 ducts, fig. 281, f, and of the uterus or urinary bladder; the 

 Lepidosiren has the peculiarly ichthyic arrangement of the anal, 

 genital, and urinal outlets. 1 



In the Dermopteri the intestinal canal is rather closely attached 

 to the back of the abdomen, though the primitively continuous 

 mesenteric fold becomes reduced in the Lampreys to filamentary 

 processes accompanying the mesenteric vessels. A similar reduc- 

 tion of the mesentery to detached membranous bands occurs in 

 the Syngnathi and Cyprinidse. 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 alons; the ventral surface of the intestines : a second mesen- 



o 



tery is continued from this part of the intestine to the ventral 

 parietes of the abdomen in the Murrena. 



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, fig. 232, it is 

 placed so far forward as to remind us of the position of the 

 excretory outlet in the Cephalopods. It is beneath the pectorals 

 in the Amblyopsis spel&us : but the more normal posterior position 

 of the vent obtains in most abdominal and all cartilaginous 

 Fishes. 



Petrified faeces or ( coprolites ' give some insight into the struc- 

 ture 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 



1 xxxin. pi. 25, fig. 1, m, n, o, I. The Branchiostoma offers no exception to this 

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

 outlet. 



