182 DE. P. CHALMEES MITCHELL ON THE 



employment among naturalists. I find, however, that such a codification was necessary 

 when I tried to arrange systematically the modifications of the characters with which 

 this memoir deals. So far as I have used them in my own work, I have found them 

 illuminating, and I offer them in the conviction that the rigorous discipline which their 

 use entails would prove of general utility. 



SYSTEMATIC DESCRIPTION. 



The Classification according to which this part is arranged is that given by Gadow in 

 Bronn's ' Thier-Reich ' (12). 



RATIT-ffil. 



CASUARII. In the Cassowaries and Emus the gut is archecentric in character. The 

 three regions are well marked ; the duodenum is wide in the Cassowary, but longer and 

 narrower in the Emu. In both, Meckel's tract is nearly circular in form, with well- 

 marked minor folds in the Emu, and, in both, with a distinct relic of Meckel's diverti- 

 culum about the middle of its course. The rectum is short and nearly straight, this 

 being the only divergence from the archecentric character and a divergence displayed by 

 the vast majority of birds. The loops are drained by the characteristic factors of the 

 common portal vein. The caeca are of moderate length as in the type. I have already 

 given a figure of the gut in the Cassowary (26. fig. 3). 



STRUTHIONES. The Ostrich (26. fig. 4) is also markedly archecentric. The diver- 

 gences from the archecentric type are very slight : the duodenum is rather longer and 

 has a minor loop situated on its distal limb ; the rectum is enormously long, being in 

 fact at least equal in length to the first two portions together; the caeca also are 

 extremely long. The factor of the duodenal vein from the caeca comes off rather further 

 from the main stem of the portal vein, and instead of running forwards within the 

 mesentery, it leaves the mesentery and in consequence must be separated when the 

 intestine is unfolded : in this condition it forms what I call a " bridging " vein. 



RHE.<E. In Hhea americana (fig. 3) the gut is archecentric, and indeed can be distin- 

 guished from that of Palamedea only in two points. Meckel's tract is somewhat 

 elongated, Meckel's diverticulum lying at its apex, and the caeca are much longer 

 relatively. The region of the cseca is drained by two " bridging " factors of the duodenal 

 vein which leave the mesentery and have to be divided when the duodenum is unfolded 

 outwards. A slight peculiarity that I have noticed in no other bird is that a factor of 

 the splenic vein (fig. 3, z) drains the proximal portion of the rectum. 



APTERTGES. The gut of the Apteryx (fig. 4), while markedly archecentric, is the 

 most modified of those among Struthious birds. The duodenum and the rectum conform 

 to type, save that the rectum is relatively shorter and is straight. The greater portion of 



