In the genus Hypsiprytnnus the stomach is as singularly complicated as in the Kangaroos, and the complication is essentially the same in both; arising from the sacculation of the pa rietes of a very long canal by a partial dispo sition of shorter bands of longitudinal fibres ; but in the Potoroos this sacculation is confined to that part of the stomach which lies to the left of the esophagus, while the right division of the cavity has the ordinary form and struc ture of the pyloric moiety of a simple stomach. The left or cardiac division is enormously de veloped ; in relative proportion, indeed, it is surpassed only by the true ruminant stomachs, in which both the rumen and reticulum are expansions of the corresponding or cardiac moiety of the stomach. The relation of the stomach of a Potoroo to that of a Kangaroo may be concisely expressed by stating that the termination of the esophagus in the former is removed from the commencement of the middle sacculated compartment to its termination.
When fluid is injected into the stomach of a dead Potoroo, it distends first the pyloric division ; it is probably by a kind of anti peristaltic action that the aliment is propelled into the long sacculated ceecum to the left of the oesophagus.
The modifications of the epiploon, as an ap pendage to the stomach, may here be noticed. In the Kangaroo it is of very moderate size, being continued loosely from the stomach to the transverse colon, but not extended beyond that part. The posterior layer lies between the stomach and the intestines, and affords a good illustration of one of the uses of the epiploon, as it evidently prevents these parts from inter fering with each other's motions. The anterior layer generally contains more or less fat. In the Petaurus the great omentum is continued from the great curvature of the stomach, and the commencement of the duodenum. In the Phalangers it is of considerable extent, and is usually loaded with fat. In the Opossums I have found it generally devoid of fat, when this substance has been accumulated in other parts. In the Phascogales and Dasyures the epiploon is of moderate size, and contains little or no fat.
Having seen that, with the exception of the Potoroos and Kangaroos, the stomach is simple in the Marsupialia, presenting only some addi tional mucous glands in the Koala and Worn bat, it is to the succeeding parts of the ali mentary canal that we have to look for those modifications which should correspond with a vegetable, a mixed, or an animal diet; and never perhaps was a physiological problem more clearly illustrated by comparative ana tomy than is the use of the ccecum coli by the varying conditions which it presents in the present group of Mammal ia.
In the most purely carnivorous group of the Marsupial order the stomach presents in the magnitude of the left cul-de-sac a structure better adapted for the retention of food than we find in the stomachs of the corresponding group in the placental series. In the most strictly carnivorous Fere, as the cat-tribe, there is a ccecum, though it is simple and short ; but in the Marsupial Zoophaga this part is entirely wanting, and the intestinal canal, short and wide,' is continued, like the intestine of a rep tile, along the margin of a single and simple mesentery from the pylorus to the rectum.
In the Entomophagous Marsupials, some of which are suspected with reason to have a mixed diet, the intestinal canal is relatively longer; the distinction of small and large in testine is established ; and the latter division commences with a simple, moderate-sized, sub clavate ccecum.
In the Carpophagous Pha !angers, whose stomach resem bles that of the predatory Da spire, the compensation is made by a longer intestine, but prin cipally by the extraordinary length of the ccecum, which it some species is twice that of the body itself.
Lastly, in the Koala, which is, perhaps, a more strictly ve getable feeder than the I'etaurists or Pha langers, the ccecum is more than three times the length of the animal, and its essential part, the inner secreting membrane is farther augmented by about a dozen longitudinal parallel, or nearly parallel, plaits, which are continued from the colon three-fourths of the way towards the blind extremity. When we reflect that the Sloth, which has the same diet and corresponding habits with the Koala, has a singularly complicated sto mach, but no ccecum, the vicarious office of this lower blind production of the digestive tube as a subsidiary stomach is still more strikingly exemplified. What then, it may be asked, is the condition of the ccecum in the Marsupials with enormous sacculated sto machs? It is in these species comparatively short and simple. In the Potoroos which scratch up the soil in search of farinaceous roots, it is much shorter than in the great Kangaroos which browze on grass. There is a slight tendency to sacculation at the com mencement of the ccecum in the latter Mar supials, by the development of two longitudinal bands (fig. 127).