144 DIGESTIVE “SV ST ii 
This feature may be termed felogyrous. 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 fourth loops are never coiled into a regular spiral, but 
rather form irregularly coiled masses, in many Pelargi, Accipitres, 
and in the Psittaci. 
We see, then, that the cycloccelous (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 cycloccelus 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 
orthoccelous 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 plagiocelous 
feature (fig. /), which was probably derived from an orthoccelous 
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 Orders 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, Rhamphastus, 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 which certain birds have 
become developed, ¢.g. the Alcedinide from a Coraciine stock, now 
modified through the acquisition of carnivorous and piscivorous 
habits. 
1 Jenaische Zeitschrift f. Naturwiss, xiii. pp. 92-117, 339-408, pls. iv.-ix. and 
xvi. ; P. Z. S. 1889, pp. 303-316, pl. xxxii,; Bronn’s hierreich. 
