2G8 DR. P. CHALMERS MITCHELL OX THE 



character of the Cathartidae. However, there is yet another condition possible among 

 these reduced caaca. Numerous observers have shown that in the caeca of many birds, 

 whether these be long or short, there occur in different regions masses of lymphoidal 

 tissue. Such masses may occur at the apex of long caeca as in the Owls, or in scattered 

 patches as in Ducks and Fowls, or in concentrated swellings as in Otis. Berry (3) has 

 recently shown that the occurrence of these lymphoid masses, in concentrated regions 

 forming vermiform appendices, or in scattered masses, is frequent in Vertebrates, and he 

 suggests that the vermiform appendix of Man is a specialized and not a degenerate 

 structure. The reduced caeca of many birds (e. g. Passerines) are quite different from 

 the obviously vestigial caeca that I have mentioned. They are regularly formed, nipple- 

 like structures, the lumen of which is greatly reduced by closely-set villi and folds. In 

 these, small glands and lymphoid tissue are abundant ; and in the Sparrow I have noted 

 the presence of abundant dark granules which are apparently matter in process of 

 excretion. I distinguish therefore in the conditions of the caeca among birds the 

 following : 



1. Arcliecentric Character. Caeca large, thin-\valled, containing food- material on 



which some process of digestion is taking place, but which may have in addition 

 the beginning of secretory or excretory function. 



2. Apocentric Characters. 



A. Caeca enlarged, partly digestive, and partly secretory or excretory. 



B. Caeca functionless, vestigial or absent. 



G. Caeca reduced but transformed solely to glandular organs, secretory or 

 excretory or both . 



PALAMEDEA. Caeca archecentric. Chauna apocentric A. 



RA.TITJE. Ctecae archecentric or apocentric A. 



COLYMBIFORMES. Arcliecentric. 



SPHENISCIFORMES. Apocentric B. but according to Gadow archecentric in some. 



PROCELLARIIFORMES. Apocentric B, occasionally one only present. 



CICONIIFORMES. Arcliecentric in the Pelecanidae, apocentric B in the Phaethontida-, 

 Sulidap, Fregatidae, and Phalacrocoracidae (in Plotus one may be absent) ; 

 apocentric B in the Scopidae, Ciconiidae, and Ardeidae, in the latter one only as 

 the normal but not invariable condition. In the Phcanicopteri urchecentric. 



ANSERIFORMES (excluding the Palamedeidae). Apocentric A, usually with large rows 

 of glands, but in Mergus apocentric B and one occasionally absent. 



FALCONIFORMES. Apocentric B, vestigial with frequent irregularities and absent in 

 Cathartidse. 



TINAMIFORMES. Apocentric A, extreme specialization in some. 



GALLIFORMES. Apocentric A in Galli and Opisthocomi ; archecentric in Tunuces. 



GRUIFORMES. In the Kallidae apocentric A; of the Gruida^ archecentric in the 

 Araminae, apocentric A in the others. In the Dicholophidae and Otididae 

 apocentric A (extremely so in Otis); in The Rhinochetidsc archecentric; in the 

 Eurypygidae apocentric, probably C ; in the Heliornithidau archecentric. 



