246 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



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

 siderable 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. 260). In a few forms (woodpeckers, 

 parrots, etc.) there is no caecum, but usually the junction of large and small 

 intestine is marked by one or two caeca (fig. 261). In some cases these caeca 

 are lined with villi, or portions may be ciliated, while the very large ca?cum of 

 the ostrich is spirally coiled. Many birds have a pocket, the bursa Fabricii, 

 of unknown functions, developed from the dorsal part of the cloaca. It arises 

 from the entodermal portion and extends forward, dorsal to the rectum (fig. 



Fig. 262. Fig. 263. 



Fig. 262. — Diagrammatic longitudinal section of the cloacal region of a duck 

 embryo at the twenty-second day of incubation, after Poindyer. ag, anal groove; c, 

 cloaca; cp, cloacal plate;/, bursa Fabricii; p, phallus, with cascal duct; sp, stercoral pouch 

 of rectum. 



Fig. 263. — Semidiagrammatic course of intestine of new-born deer Cervus canadensis, 

 after Weber, c, caecum; d, duodenum; co, colon;/, jejunum; m, mesentery. 



262) later connecting with the ectodermal (proctodeal) part of the cloaca. 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 caeca in some eden- 

 tates, while some edentates, bats, carnivorous mammals and many whales 

 lack either caecum or valve. The caecum is larger in the herbivorous forms 

 and frequently there is a relation between the development of caecum and 

 stomach. The caecum becomes enormous in certain rodents and marsupials 

 (sometimes longer than the body) and plays an important part in digestion, 

 being sometimes lobulated or furnished with internal folds, those of the rabbits 

 being arranged in a spiral manner. In man and the anthropoids and some 

 other forms, as is well known, the distal part of the caecum degenerates to a 



