ALIMENTARY SYSTEM. 



543 



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 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 in- 

 testine. 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 valve of skate. 



After T. J. 

 Parker. 



significance arises as a vascular diverti- 

 culum 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 peri- 

 toneum, the membrane 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 communication with the exterior. Excepting 

 mouth cavity and cloaca, the gut is lined by endoderm. 



Respiratory system. The first apparent gill-clefts the 

 spiracles open dorsally behind the eyes. Each contains 

 a rudimentary gill on the anterior wall, supported by a 



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