ALIMENTARY SYSTEM. 



493 



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, 

 communicate 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 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 con- 

 tains an internal spiral fold — which greatly 

 increases the absorptive surface. 



Fig. 208. — 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 Mam- 

 mals, for instance, the result is a coiled intestine. 

 But in Elasmobranch fishes the coiling or twist- 

 ing 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- 

 brane which lines the body cavity ; the rest of the gut lies 

 freely. Rectum, ureters, and genital ducts all communicate 

 with the exterior through the common terminal chamber 

 or cloaca. An abdominal pore opens on each side of the 

 cloacal aperture, and puts the body cavity in direct com- 

 munication with the exterior. Excepting mouth cavity and 

 cloaca, the gut is lined by endoderm. 



Respiratory system.— The first apparent gill-clefts — the 



