INTESTINAL TKACT OF BIEDS. 221 



Meckel's divei-ticulum, but towards the extremity of this loop the place of attachment 

 of the yolk-sac was marked hy a strong remnant of a ventral mesentery. In a third 

 adult this mesentery ran to a very small vestige of Meckel's diverticulum. The most 

 distal loop of IMeckel's tract was wide, and closely applied to it were the pair of large 

 caeca, drained partly by a "bridging" factor of the duodenal vein. The rectum is 

 unusually long, retaining the archecentric condition of being thrown into minor folds. 



It is plain that the condition of the intestinal tract in this group is markedly apocentric, 

 except with regard to the rectum. From this point of view the Ojjisthocomi do not find 

 a natural place among the Galliformes — first, because in these latter the general condition 

 of the gut is archecentric ; and second, and more important, the small degree of apocen- 

 tricity displayed among some of them consists of a peculiar expansion of tlie distal 

 portion of Meckel's tract, and there is no trace of this feature in Opisthocomus. The 

 apocentricity of Opisthocomus may be compared first with that of the Tinamus. If 

 the very small loop seen just distal to Meckel's diverticulum in H/ii/iichofiis {&g. 3G) were 

 ])rolonged, carrying with it the diverticulum, the condition in Opisthocomus would be 

 reached. A second suggestion of relationship is, as I have already pointed out, given by 

 comparison with the form of the tract in Ftei-ocles and the Pigeons. In Pterocles 

 (26. fig. 18) tlie condition of Meckel's tract and of the long caeca is extremely like that in 

 Opisthocomus. the most notable diff'erence being that in Opisthocomus the axial loop bearing 

 the diverticulum is slightly twisted. In the Pigeons (26. fig. 19) the twnsting of the axial 

 loop may be carried much further and the caeca are reduced. AVhen I first made this 

 comparison of the gut in these three groups, I accejited the common view that eutaxy or 

 quintocubitulism was a primitive condition, and diastaxy or aquintocubitalism a derived 

 condition, and it was with hesitation that I jjvit Opisthocomus, an eutaxic form, between 

 Pterocles, a diastataxic form, and the Columbidfe, then believed to be diastataxic. Since 

 then I have shown that the eutaxic condition is probably a muhiradial apocentricity 

 derived independently from the diastataxic, archecentric condition of the wing. Moreover, 

 I have shown (28) that among the ColumbidEe eutaxic forms occur. There is therefore 

 no difficulty in the way of supposing that the gut forms in the I'igeons and in Opistho- 

 comus are derivatives of the condition in Fteroclcs. In the paper in which I described 

 the intestinal tract of Opisthocomus (27) I pointed out also that there were resemblances 

 between the gut of Opisthocomus and the gut ot the Cuculidui, thereby recjilling Garrod s 

 (13) suggested relationship between Fowls, Opisthocomus, and Cuculidae. It is true that 

 among the Cuculidae Meckel's tract is thrown into three loojis, but an examination of a 

 larger number of Cuculidae has shown me that important differences distinguish the three 

 loops in Opisthocomus and the loops in Cuculidae. 



Taking the Galliformes as a whole, it appears that tlie form of the gut in the Turnices 

 is archecentric; in the Galli it is still archecentric, but with a tendency to a special mode 

 of apocentricity ; in the Opisthocomi it is markedly apocentric, and the apocentricity is 

 quite different in kind from that found among the Galli, but with marked resemblances 

 to the condition in Fterocles and the Pigeons. 



SECOND SEBIES. — ZOOLOGY, VOL. VIII. 84 



