522 



P7SCES FISHES. 



tympanum. Within the vestibule are calcareous otolithic par- 

 ticles surrounded by a jelly. 



(c) The nasal sacs are cup-like cavities with plaited walls. 



(d) The sensory tubes are best seen on the ventral surface, where 



they lie just under the skin. At their internal ends lie ampulla?, 

 containing sensory cells. At their outer ends there are pores. 

 It is probable that they are organs partly of touch, and partly 

 of " chemical sense." 



Alimentary system.- -The mouth is a transverse aperture; 

 the teeth borne by the jaws are numerous, and those worn 

 away in front are replaced by fresh ones from behind ; naso- 



buccal grooves connect the nostrils with 

 the corners of the mouth ; the spiracles, 

 which open dorsally behind the eyes, com- 

 municate with the buccal cavity ; from 

 the gullet five gill-clefts open ventrally on 

 each side. The stomach, lying to the left, 

 is bent upon itself; the large brownish 

 liver is trilobed, and has an associated 

 gall-bladder, from which the bile-duct ex- 

 tends to the duodenum the part of the 

 gut immediately succeeding the stomach ; 

 the whitish pancreas lies at the end of the 

 duodenal loop, and its duct opens opposite 

 the bile-duct. The intestine is exceed- 

 ingly short, but it contains an internal 

 spiral fold which greatly increases the 

 absorptive surface. 



The development of this spiral intestine is of general interest. The 

 well-nourished gut grows quickly, but its increase in calibre is hindered 

 by the peritoneal mesodermic sheath, and the growth is expressed in 

 an internal invagination or fold. But as the growth continues in 

 length as well as in calibre, and as the gut is fixed at both ends, 

 twisting or coiling or both must result. In Mammals, for instance, the 

 result is a coiled intestine. But in Elasmobranch fishes the coiling or 

 twisting takes place within the peritoneal sheath, not along with it. 

 In the case of the skate and some other Elasmobranchs, close twisting 

 occurs, and the so-called spiral valve is mainly due to the fusion of the 

 walls of adjacent twists. 



A small "rectal gland" of unknown significance arises as a 

 vascular diverticulum from the end of the gut. The end of 

 the gullet and the anterior portion of the stomach and the 

 rectum are supported by folds of peritoneum, the mem- 



FiG. 250. Spiral 

 valve of skate. 

 -After T. J. 

 Parker. 



