DIGESTIVE ORGANS. 229 
known, but it apparently corresponds to the ceca of the higher groups. 
In the ‘small’ intestine is the spiral valve which has two forms, both 
leading to increase of surface. In most species a fold, carrying blood- 
and lymph-vessels, arises in a spiral line from the wall of the tube, and 
its free edge projects into the lumen like a spiral stairway (fig. 231). 
In a few forms (Carchariide, Galeocerdo) the 
line of origin of the fold is straight and its free 
margin is coiled like a roll of paper (fig. 232). 
In the large intestine rectum and cloaca are 
recognized, the cloaca being that part which 
receives the ends of the excretory and repro- 
ductive ducts and thus is both digestive and 
urogenital in character. 
Ganoids and dipnoi (figs. 230, 233) also have the 
intestine nearly straight and a spiral valve, least 
developed in Lepidosteus. In the teleosts the canal 
may be straight (fig. 227) or may make more or fewer 
coils, the predaceous species being simplest, while in 
the mullet (Mugil) there may be 13 or 14 turns. In ‘ 
the teleosts the line between small and large intestine \ 
is often marked by an ileo-colic valve and a few species 
have a cecum or rectal gland. A spiral valve rarely q 
occurs in teleosts and a cloaca is never found. Ina { ] 
few teleosts, in correlation with the translation of the |, @ 
ventral fins, the anus may lie in front of the pectoral 
girdle. 
The intestine is straight in the cacilians, has a 
few coils in the perennibranchs and more in the sala- ( 
manders, while the anura have a greatly convoluted Br 
intestine. (Reference has already been made to the Fic. 233.—Digestive tract 
differences between the intestines of the larval and pated (Stenostomus chr ysops 
ae —FPrinceton 296). 6d, bile 
adult frogs (p. 228). The line between small and duct; gd, gall bladder; J, liv- 
large intestine is frequently marked in the amphi- ¢t; /, large intestine: pc, 
bians by an ileo-colic valve and in a few forms bye Gees; Sh small ip 
(Rana, Salamandra) there is a rudimentary cecum. 
The rectum is larger than the rest of the intestine and a cloaca is always present 
in the amphibia. 
The reptiles have the intestine coiled (nearly straight in amphisbenans) and 
usually of about the same diameter throughout. Small and large intestine are 
separated by an ileo-colic valve, and except in crocodiles a caecum is usually present, 
while a cloaca constantly occurs. The spirally twisted coprolites of the ichthyo- 
saurs have been supposed to indicate the existence of a spiral valve, but since in 
other groups the feces are formed in the rectum, this is not conclusive. 
