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-bJadder, 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 w2thzz 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 ces hh 
significance arises as a vascular diverti- —After T. J. 
culum from the end of the gut. The end Parker. 
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 
