DR. P. CHALMERS MITCHELL ON THE 



Meckel's tract forms a large circular fold with Meckel's diverticulum at its central point, 

 opposite the origin of the middle mesenteric vein, as in the archecentric type. But the 

 posterior portion of the tract, that to which the very long caeca are attached, is specialized 

 into a distinct minor loop. This I call the supra-duodenal loop, because, in the natural 

 condition of the intestine, it lies directly above the duodenum and is drained by factors 

 of the duodenal vein. This loop in Apteryx is the first appearance of a specialized 

 structure to which I shall have to direct repeated attention. 



When correction has been made for the obviously homoplastic modifications in the 

 intestinal tract of Ratites that is to say, when a reduction has been made in the case of 

 the larger forms for size and in the case of the herbivorous forms, it is plain that while 

 all are archecentric, the Casuarii are by far the least modified, and that in this as in 

 many other characters they deserve the position assigned them as extremely primitive 

 types. Struthio and Rhea are more modified, and the modification is in the same 

 direction in each. Apteryx stands somewhat apart from the others, the specialization of 

 the supra-duodenal loop being well marked in it- The general character in all, however, 

 is so plainly archecentric that no argument can be drawn from the intestinal tract for or 

 against the polyphyletic nature of the group. Such similarities as they present to one 

 another or to other groups are simply part of the common inheritance of all birds. 



COLYMBIFORMES. 



COLYMBI. I have examined the Great Northern and Red-throated Divers (Colymbus 

 glacialis and C. septentrionalis), and have already figured the intestinal tract (26. fig. 5). 

 The duodenal Icnp is straight and normal; the rectum is straight and very short. 

 Meckel's tract is pulled out into a series of short, straight, symmetrically arranged loops 

 with the Meckel's diverticulum nearly at the middle of its course, and with a well 

 specialized supra-duodenal loop drained by a bridging factor of the duodenal vein. The 

 three main factors of the portal vein are typical, and the caeca are of moderate length. 

 It is clear that the Divers present a certain degree of apocentricity in the gut. The 

 fish-eating habits have lengthened the tract, but the increase of length takes place not 

 by a series of irregular loopings but in a definite fashion. 



PODICIPEDES. In the Crested Grebe, Podicipes cristatus (fig. 5) the duodenum is a 

 large loop somewhat wider distally. Meckel's tract is still more specialized than that of 

 the Colymbi. It presents first a long minor loop, then a short loop, then a long loop, 

 on which, from the arrangement of the blood-vessels, I suppose the yolk-sac to have lain, 

 although it is not represented by a Meckel's diverticulum ; then follow two short loops, 

 and then a very large supra-duodenal loop with characteristic bridging vein. The caeca 

 are relatively short, but wide and functional, and the rectum is short and wide. 



In Tachybaptes fluviatilis, the Little Grebe (fig. 6), the apocentricity seen in the 

 Crested Grebe is carried further in two points, otherwise the character of the tract is 



