ALIMENTARY CANAL OF SKATE 



60 1 



auditor}' capsule is covered only by the skin, forming a kind of 

 tympanum. Within the vestibule are calcareous otolithic 

 particles 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 

 ampullar, 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 aper- 

 ture ; 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, communicate with the buccal cavity ; 

 from the gullet five gill-clefts open ven- 

 trally 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 extends 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 exceedingly short, but 

 it contains an internal spiral fold — 

 which greatly increases the absorptive 

 surface. 



Fig. 338. — Spiral 

 valve of skate. 

 —After T. J. 

 Parker. 



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," partly lubricant, partly blood- 

 making, arises as a vascular diverticulum from the end of 



