The Digestive System 155 
The small intestine has heavy muscular walls 
whose histological structure will be described else- 
where. It opens abruptly, without any indication 
of a czecum, into the large intestine or rectum. 
The rectum, r, is of about twice the diameter of the 
small intestine, though this, of course, varies with 
the amount of fecal matter it contains; it is nearly 
straight and possesses much thinner walls than 
the small intestine, though this, again, varies 
with the state of collapse or distention. 
At the posterior end of the rectum is a heavy 
sphincter valve separating that part of the intestine 
from the cloaca. 
The cloaca, c, is widest anteriorly where it is 
about as wide as the rectum; it gradually diminishes 
in diameter caudad, and appears flattened later- 
ally. Its wall has the same general structure as 
the rectum, as will be described below. The 
mucous membrane posterior to the openings of the 
genital ducts is thrown into a more or less com- 
plete, ring-like transverse fold (Fig. 55 G.). In 
some species there may be a second, half-ring-like 
fold in the dorsal wall caudad to the more complete 
ring. The cloaca is divided by this fold into a 
larger anterior portion, g, and a shorter posterior 
portion, h; in the former the mucous membrane is 
thrown into a large number of small folds that in 
places form a network; in the latter the mucous 
membrane has a hard, thick epithelium, with a 
smooth surface and only a few longitudinal folds. 
