442 DE. P. CHALMEBS MITCHELL ON THE 



Systematic Description. 



Class MAMMALIA. 



Sub-Class MONOTEEMATA. 



Order MONOTEEMATA. 



Ornithorliynchns (fig. 1). 



I am indebted to my friend Dr. Wilfrid Harris for the opportunity of examining the 

 intestinal tract of a specimen of Platypus. 



There is no specialised duodenal loop. Meckel's tract is nearly circular in disposition, 

 being suspended in very regular minor loops at the circumference of a circular expanse 

 of mesentery. The caecum (fig. 1, C) is of moderate length and slender, but is hollow 

 to the tip. There is a well-marked colic loop (fig. 1, C.L.), and the rectum is straight. 

 The veins show no feature of special interest. A single main tributary of the portal 

 vein drains the whole area of Meckel's tract and receives a tributary from the colic loop 

 and a posterior mesenteric vein from the rectum, the latter being shown divided in the 

 figure at w. The original position of the mesentery is little disturbed, but in association 

 with the outgrowth of Meckel's tract and the colic loop a certain amount of twisting 

 has taken place, and in the undisturbed condition the colic loop is attached closely to 

 the duodenal portion. 



Echidna (fig. 2, p. 444). 



There is no specialised duodenal loop. Meckel's tract is circular, as in Ornitho- 

 rliynehus, but is thrown into more complex minor loops, the length of which, as well as 

 the number, is rather underestimated in the diagram. The caecum (fig. 2, C) is very 

 short, much shorter than in Ornithorliynehus, but is hollow to the tip. The colic loop 

 (fig. 2, C.L.) is less differentiated from Meckel's tract and is itself thrown into a series 

 of minor folds. The rectum is short and relatively wider ; it runs a straight course, 

 following the dorsal mesentery. In the figure it is drawn twisted round. The portal 

 vein is diagrammatically simple : a single large vessel receives a number of regularly- 

 disposed tributaries from Meckel's tract, and is joined posteriorly by a rectal or 

 posterior mesenteric vessel. 



The patterns of the gut in the Monotremes are not much removed from a primitive 

 reptilian simplicity. At first sight, the figures recall the arrangement in many Birds, 

 but there are two marked differences. In the first place, practically every Bird has a 

 specialised duodenal loop; this loop in the Monotremes, as in Mammals generally, is 

 not separated off from the proximal portion of Meckel's tract. Secondly, the posterior 



