INTRODUCTORY 21 



The convolutions into which the small intestine is thrown, 

 for the purpose of securing a greater length of gut, and con- 

 sequent increased digestive surface, are of considerable value 

 for taxonomic purposes, as Dr. Hans Gadow and Dr. Chalmers 

 Mitchell have successively shown (p. 436). 



But the length of the gut, and the thickness of the walls 

 thereof, are largely determined by the nature of the food to be 

 assimilated. Thus, in purely frugivorous and insectivorous birds 

 the gut is very short, while it reaches its maximum length in 

 species which feed upon fish, carrion, grain and green vegetable 

 matter. 



The lumen of the gut in short-gutted forms is generally very 

 wide, while in piscivorous birds the walls of this canal are 

 always very thick — a possible contrivance to lessen the danger 

 of perforation by fish-bones. 



At the junction of the small and large intestines there may 

 be found a pair of caeca or blind diverticula, guarded by a 

 valve which allows the semi-faecal matter to pass into these 

 pouches, but prevents any backward movement into the small 

 intestine (ileum). 



In some birds, such as the African Ostrich, Rhea (111. 7, p. 23) 

 and the Gallinae these caeca are of relatively enormous size ; 

 they are also very large in some of the Wading-birds (Limicolse), 

 in Owls and in Night-jars, Rollers, Bee-eaters and Cuckoos which 

 are insectivorous. 



In fish-eating birds they are of relatively small size, and in 

 some species are quite degenerate. In some species they are 

 reduced to mere wart-like bodies, of which one may be wanting 

 altogether, e.g.. Herons and Petrels, or both may have com- 

 pletely disappeared as in many Pigeons, Parrots, Kingfishers 

 and the Swifts, for example. 



In Struthio and Rhea the aggregate volume of these ap- 

 pendages, as Dr. Hans Gadow has pointed out, may surpass 

 that of the rest of the intestinal canal. 



These pouches do not present any great range in the 

 matter of shape. As a rule they are cylindrical. But in the 

 Owls they assume the form of inverted Florence-flasks; while 

 in the Martineta Tinamou {Calodromas elegans) they assume a 

 truly remarkable form, having the semblance of a bunch of 

 grapes! (111. 7, p. 22). 



