22 A HISTORY OF BIRDS 



The caeca appear to function as digestive organs when 

 fully developed ; but in some cases they seem to have assumed 

 a new character. Such instances appear to obtain in many 

 cases where these organs would seem, judged solely by their 

 small size, to be degenerate, as, for example, in Passeres. But 

 here, as Dr. Chalmers Mitchell has pointed out, the walls of 

 these extremely reduced organs contain lymphoid tissue, though 

 what function they serve, whether secretory or excretory, is yet 

 unknown. The caeca of the Owls similarly, at their ends, con- 

 tain masses of lymphoid tissue, while in Ducks and Fowls this 

 occurs in scattered patches. Thus, while some reduced caeca 

 are certainly degenerate and functionless, others, though re- 

 duced, still play a more or less important r61e in the meta- 

 bolism of the body. 



But the presence of these organs presents some curious 

 anomalies, which seem to show that although their develop- 

 ment is correlated with certain kinds of diet, this relation is by 

 no means always maintained. The diurnal birds of prey and 

 the Owls afford the most striking illustration of this peculiarity. 

 Both are now flesh eaters, and the former have apparently in 

 consequence lost them, yet in the latter they are of large size. 

 Some of the Owls, it is true, still partake freely of insect food, 

 but so also do many of the smaller Falcons, which indeed live 

 exclusively on this diet, yet they, like their larger brethren, are 

 minus these organs. One must assume that they were lost 

 before the insect diet became fixed, and so could not be re- 

 developed. But it is clear that they are not essential to the 

 digestion of insect food, and equally clear that the substitution 

 of a carnivorous diet need not bring about their dissolution. 



The rectum, like that of reptiles, terminates in a cloaca di- 

 vided into a series of more or less distinct chambers, the copro- 

 daeum, urodaeum and proctodaeum, into which last opens the 

 " Bursa Fabricii," an organ of unknown function, peculiar to 

 birds, and largest in nestlings ; into the urodaeum open the 

 kidney and the genital-ducts. The coprodaeum retains the 

 faecal and urinary matter until ready for expulsion. 



The Circulatory System 



The main features of the circulatory system may be very 

 briefly summarised. 



