(.10 



Comparative Morphology of Chordates 



of a circular constricting muscle (sphincter) situated in an inward- 

 projecting circular fold of the wall of the tube. 



One of tli<' several ways whereby the secreting and absorbing 

 surface of the intestine is increased is the formation of pouches or 

 ceca protruding outward from the main tube. In Amniota the colon 

 commonly produces one or more ceca which, with rare exceptions, 

 arise just behind the ileocolic passage. A single short colic cecum 



Fig. 186. Alimentary canal of rat (Mus 

 decumanus), the greater part of the small 

 intestine being omitted, (c) Colon; (cm) 

 cecum; (d) duodenum; (i) ileum; (o) 

 esophagus. (Courtesy, Flower and Lydek- 

 ker: '* Introduction to the Study of Mam- 

 mals," London, A.. & C. Black, Ltd.) 



appears in most reptiles, but there is none in crocodilians. Birds usually 

 have two ceca at the anterior end of the short posterior intestine, and 

 these may attain great length (p. 530). In mammals there is usually 

 but one cecum (Figs. 485, 486). It is situated in close relation to the 

 ileocolic valve, which, in fact, may become an ileocecal valve. But in 

 rare cases (e.g., the six-banded armadillo) there are two ceca, while a 

 cecum is entirely lacking in the pangolin (Manis), the South American 

 giant anteater (Myrmecophaga jubata), sloths, some cetaceans, some 

 carnivores, and most bats. The Asiatic coney, Hyrax,has a moderately 

 large cecum in the usual position and a pair of capacious ceca at a more 



