30 STRUCTURE AND CLASSIFICATION OF BIRDS 



folded intestine for a greater or less extent ; thus in the 

 penguin a good deal of the intestine retains the early 

 system of folding. But this bird is remarkable for the 

 extraordinary complications of the duodenal loop, which has 

 become enormously lengthened, and, in Eudyptes, thrown 

 into a series of secondary loops, which in Aptenodytes 

 are arranged in a spiral a convergent resemblance, thinks 

 MITCHELL, to Haliaetus (see above, fig. 13). Platalea, 

 which, possessing as it does the complete muscle formula of 

 the leg, must be regarded as a primitive type among the 

 storks, has the greater part of the small intestine disposed 

 in the primitive fashion ; the duodenal loop is, of course, 

 distinct, and there is also a well-marked long loop just 

 before the large intestine. Haliaetus albicilla, to which 

 reference has just been made, is less divergent from the 

 primitive form than many other birds, and Lanis marinus 

 has preserved considerable traces of the same. Among the 

 most specialised birds from the present point of view are 

 the parrots (see fig. 14) . 



In Ara, at any rate, there is nothing left of the original 

 primitive folds of the intestine ; the whole of its course is 

 disposed into six specialised loops. The owls and the Capri- 

 mulgidse appear, according to MITCHELL, to have preserved 

 much of the primitive short, convoluted loops of the lower 

 birds, and so, though to a less extent, has Corythaix. The 

 very short intestine of the Passeres, represented here by Parus 

 major, is not so easy to understand, but its general appearance 

 is that of a primitive gut. Some further details will be found 

 in the succeeding chapters, under the different families to 

 which they refer. 



The caeca lie at the commencement of the large intes- 

 tine, which indeed can be, as a rule, only differentiated from 

 the small intestine by the break thus caused. This is, how- 

 ever, not invariably the case, for in Stntthio (see fig. 12, p. 28) 

 there is a distinct break between the two sections of the 

 gut, quite independent of the caeca, which can be readily 

 seen from an inspection of the figure cited. The caeca are 

 among the most variable organs of birds. Not only are they 



