230 



COMPARATIVE MORPHOLOGY OF VEGTEBRATES. 



The intestine is longer in the birds than in the reptiles, but there is considerable 

 difference in the group in this respect. The great increase comes in the colon which 

 is coiled in different ways, which may be reduced to seven plans or combinations 

 of loops and spirals (fig. 234). In a few forms (woodpeckers, parrots, etc.) there 



A 



FIG. 234. -Types of coiling of the intestines of birds, after Gadow. A, isocoelous; 

 B, anticoelous; C, antipericoelous; D, isopericcelous; E, cyclocoelous; F, plagioccelous; G, 

 telogyrous; p, pylorus. 



is no caecum, but usually the junction of large and small intestine is marked by one 

 or two caeca (fig. 235). In some cases these caeca are lined with villi, or portions 

 may be ciliated, while the very large caecum of the ostrich is spirally coiled. Many 

 birds have a pocket, the bursa Fabricii, of unknown functions, developed from the 



FIG. 235. Alimentary canal of Chauna, after Mitchell, c, caeca; I, large intestine; p 

 proventriculus; pv, portal vein; rv, rectal vein; s, small intestine; v, remnant of vitelline 

 duct. 



dorsal part of the cloaca. It arises from the ectodermal (proctodeal) portion and 

 extends forward, dorsal to the rectum (fig. 236). In some cases it degenerates in 

 the adult. 



The limits of large and small intestine in the mammals are usually marked by an 

 ileo-colic valve and a single caecum, but there are two cseca in some edentates, 

 while some edentates, bats, carnivorous mammals and many whales lack either 

 caecum or valve. The cascum is larger in the herbivorous forms and frequently 



