INTESTINAL TRACT OF BIRDS. 269 



CHARADRIIFORMES. 



LimicoloK. In the Glareolidae archecentric ; in the Chionidae, Thinocoridse, and 

 Charadriidae apocentric A, but in some of the latter family apocentric C. In 

 the Parridae, Alcidse, and most of the Laridse apocentric B and sometimes 

 practically quite absent, but in Stercorarim archocentric. 



Pterocletidce. Apocentric A. 



Columbida;. In the vast majority apocentric B, occasionally one and very often 

 both absent, but in Columba livia and Columba cenas traces of glandular 

 structure and so apparently apocentric C. 



CUCDLIFORMES. 



Cuculi. Archecentric to apocentric A in the Cuculidse ; in the Musophagidae 

 apocentric B, and usually, if not always, quite absent. In the Psittaci 

 apocentric B. 

 CORACIIFORMES. In the Coraciidae, Momotidae, and Meropidae archecentric ; in the 



Alcedinidae and Upupidae apocentric B. 

 Striges. Apocentric A. 

 Caprimnlgi. Archecentric. 

 Trogones. Archecentric. 

 Cypseli, Colii, and Picl are apocentric B. 

 PASSERIFORMES. All that I have examined are apocentric C. 



Gadow (12. pp. 688-693) has made a very careful study of the relations between the 

 character of the caeca and the nature of the diet, and I have gained further information 

 from Finn (8) and from Evans (7), and from the keepers at the Zoological Gardens as to 

 the diet of birds. There is very far from an exact correlation between the apocen- 

 tricities and particular diets, but there is enough to make it plain that many of the 

 modifications are homoplastic ; that is to say, that they are multiradial apocentricities 

 on which no argument as to affinity can be based. 



Archecentric Type. The diet in these forms is by no means uniform, but there is 

 a preponderance of feeding on insects and caterpillars, grubs, and so forth ; 

 in fact, of what may be taken as more typical reptilian diet. 



Apocentric Type A. A very large number of birds displaying much enlarged ca3ca 

 live on vegetable diet in which cellulose bulks largely, and it is to be remembered 

 that the digestion of cellulose occurs in the large caeca of some mammalia. 

 Apocentric Type B. Carnivorous diet, piscivorous diet, and especially frugivorous 

 diet are specially associated with the degeneration of the ca?,ca to functionless 

 vestiges, or even with the complete disappearance of the caeca. 



Apocentric Type C. When it is remembered that the vast majority of the Passeres 

 display this type, and that yet almost every possible form of diet is found among 

 them, it is at once clear that diet is no complete explanation of the character of 

 the caeca. I am more inclined lo think that type C is to be associated with a 

 high development of the Avian structure, and that it is a peculiar secretory or 

 excretory organ. 



SECOND SERIES. ZOOLOGY, VOL. Till. 40 



