144 DIGESTIVE SYSTEM 



This feature may be termed telogyrous. With the duodenum this is 

 very rare, it then invariably forms a right-handed spiral, e.g. in 

 Buceros, Ciconia, and Milvus ; the duodenum is more irregularly 

 twisted in certain Pelargi and Accipitres. The ends of the second, 

 third, and foixrth loops are never coiled into a regular spiral, but 

 rather form irregularly coiled masses, in many Pelargi, Accipitres, 

 and in the Psitfaci. 



We see, then, that the cyclocoelous (meso- or telogyrous) feature 

 by itself cannot be taken as a character which indicates the affinity 

 of the larger groups or Orders of Birds, unless we take the mode 

 of development of these concentric convolutions into consideration. 

 In fact, the cyclocoslus formation is the highest mode of stowing 

 in the smallest compass that portion of the gut which had to be 

 increased in length, the relative length of the mid -gut being 

 dependent upon the nature and composition of the food. In strictly 

 orthocoelous birds the increased length of the gut causes the formation 

 of secondary folds anywhere between the previously existing loops, 

 whereby frequently a very irregular arrangement of all the convolu- 

 tions is caused. A similar process has produced the plagiocoelous 

 feature (fig. /), which was probably derived from an orthocoelous 

 basis. 



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 interstitial growth, turns 

 into a spiral ; in this way the other loops will undergo the least 

 possible disturbance. 



It is not necessary to give here a long and detailed enumeration 

 and description of the intestinal convolutions as they occur in the 

 numerous Oi'ders and Families of birds, because this has been done 

 elsewhere.^ 



Secondary shortening and widening of the gut (owing to the 

 assumption of frugivorous habits) may reduce the number of loops, 

 and may render the original arrangement quite untraceable, as in 

 Carpophaga, Ekamphastus, and Manucodia. When a bird has acquired 

 strictly piscivorous habits, the gut is considerably lengthened and 

 narrowed and may, as in Pandion and in Haliaetus, render the old 

 formation quite unrecognizable. These are, however, exceptions, 

 which are not numerous ; as a rule the lengthening of the pre- 

 existing loops and the additional intercalation of new ones does not 

 disturb the typical formation, but rather throws interesting lights 

 upon the lines of new departure along Avhich certain birds have 

 become developed, e.g. the Alcedinidse from a Coraciine stock, now 

 modified through the acquisition of carnivorous and piscivorous 

 habits. 



1 Jenaische Zeitschrift f. Naturwlss, xiii. pp. 92-117, 339-403, pis. iv.-ix. aud 

 xvi. ; P. Z. S. 1889, pp. 303-316, pi. xxxii. ; Bronn's Tkierreich. 



