446 DE. P. CHALMERS MITCHELL ON THE 



In the Opossums the duodenal loop is well differentiated. Meckel's tract is a nearly 

 regular circular stretch of gut at the circumference of its mesentery, and is thrown into 

 only comparatively slight minor loops, but at its distal extremity, towards the recurrent 

 part of the circle, it is nearly straight, as in most Birds and Mammals. The single 

 caecum is very large, wide, and long (C), and marks the passage of the gut into the 

 hind-gut, which is not differentiated into colon and rectum and is only slightly 

 expanded in length, being thrown into a few minor loops. The portal system is 

 extremely simple, consisting of a single anterior mesenteric branch which curves round 

 Meckel's tract, receiving tributaries from the duodenum and from the circular expanse. 

 One of these tributaries, specially enlarged, drains the caecum, and the rectal or 

 posterior mesenteric vein joins the anterior mesenteric at the edge of the mesentery. 

 Probably this pattern is as nearly primitive as is to be found amongst Mammals. 



The hind-gut with its mesentery is represented in the figure (fig. 3) as being twisted 

 over, the specimen having been put in that position for convenience in drawing ; but 

 the rectal vein marks the line of the original straight dorsal mesentery, and it will be 

 seen that the gut displays in a simple form a duodenal fold, a circular Meckel's tract, 

 and a hind-gut in almost diagrammatic simplicity. The posterior mesenteric vein is in 

 its primitive condition as the great longitudinal vein of the gut, whilst the anterior 

 mesenteric departs from its primitive simplicity only in so far as its original main 

 termination on the yolk-sac has dwindled, with the disappearance of the yolk-sac, into 

 a simple tributary, whilst the main vessel appears to follow the contour of the gut. 



Family Dasyurid^e. Dasyurus viverrinus (fig. 4), Phascogale penicillata. 



In these small carnivorous forms the duodenal fold is little differentiated, and 

 Meckel's tract is a very simple circular coil thrown into evenly disposed minor loops. 

 There is no trace of a caecum, and the hind-gut, which is undivided into colon and 

 rectum, is relatively short and simple. The portal system appears to correspond closely 

 with that in the Opossum, except for the absence of the caecal branch. 



It is impossible not to regard the presence of a caecum as a primitive feature in 

 Mammals, and hence in the Dasyuridae, as in most of the Insectivora, wdiich the 

 Dasyuridae resemble in habits, the absence of the caecum must be regarded as secondary. 

 It is equally plain that the gut is degenerate from an extremely simple type. 



Family Notoryctid^e. 

 I have not been so fortunate as to examine an example of this rare type. 



Family PerameliDoE. Peragale lagotis (fig. 5, p. 448). 



The gut of the Bandicoot is less degenerate than that of the Dasyuridae. The 

 duodenal loop is well separated ; Meckel's tract is relatively short, but has the normal 



