FOOD AND ITS UTILISATION 107 



which contains relatively little undigested or indigestible 

 food. Another reason is that many birds are very selective 

 in their eating, and do not take much that they cannot use. 

 A third reason for the extraordinary shortness of the large 

 intestine may perhaps be found in the habit owls and many 

 birds of prey have of casting pellets of indigestible materials 

 from their crop. 



The twofold problem of a food-canal is to digest and 

 absorb, and the looping of the bird's ileum is a fine example 

 of close packing. Great length is made possible without 

 great bulk. Professor Newton made a special study of the 

 intestinal looping, and some of his results are very instructive 

 (1893, p. 140). 



" In early embryonic stages the intestinal canal is a 

 straight tube ; but, as its growth proceeds far more rapidly 

 than that of the body-cavity, it is necessarily thrown into 

 folds or loops. Moreover, since it is suspended from the 

 vertebral column by the mesentery, or lining of the body- 

 cavity, its several folds are thereby connected with one 

 another in various ways, and their number and shape 

 depend to a great extent upon the space available in the 

 cavity, as well as upon the shape, size, and position of the 

 stomach and neighbouring organs ; but the various ways 

 in which the small intestine is stowed away in different 

 birds exhibit types so definite and constant that they cannot 

 be considered accidental or meaningless features." 



The fact is that the mode of looping is very diagnostic 

 of relationship ; it has great " taxonomic value." In related 

 birds there are similar patterns, and some of the divergent 

 cases are exceptions that prove the rule, for in some cases 

 they can be correlated with disturbing peculiarities of diet. 

 It is very interesting to find that old loops may be lengthened 

 out and new ones intercalated without disturbing a typical 

 intestinal pattern characteristic of families or groups of 

 famihes. " The highest and perhaps newest mode of 

 stowing an increased amount of intestinal length is that in 

 which one of the folds already existing is lengthened and, 

 owing to its intestinal growth, turns into a spiral ; in this 



