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Show 1902.] ADAPTATIONS IN DIPROTODONT MARSUPIALS. 17 transversely very much elongated, but longitudinally quite narrow, meshes. The figure (text-fig. 4) represents the blind end of the caecum from the last sphincter, and shows also the transverse plicae described above. W h e n the caecum is filled the blind end seems to taper to a point not unlike a vermiform appendix. The colon is thin-walled and smooth, only showing, in places where it is contracted, longitudinal folds, which become effaced by stretching. From this description it will be evident that the caecum of Trichosurus is an organ which has become to a considerable degree specialized for digestion (and re-absorption). When the chyme passes through the terminal portion of the ileum it becomes mixed with the secretion of the glands of that intestinal tract, to which is added, when it enters through the ileo-caecal valve, the secretion from the caeco-colic glandular patch. Thanks to the sphincters and the well-developed muscular coat of the caecum, the food can be moved backwards and forwards, or retained in the caecal divisions, and then be subjected to the action of the glands through the development of an increased surface due to the transverse plicae. This great specialization gives a satisfactory explanation why the caecum of Trichosurus does not need to grow out to such a size and attain such a capacity as that of the Koala. It might be questioned whether any proofs can be given to show that cellulose is really decomposed and digested in the caecum of this animal. It may then, firstly, be referred to the peculiar structural specialization described above; secondly, it may be stated that the contents of the colon which have passed the caecum seem to indicate such a digestion. There may still be recognized remains of the most resistant parts of the vegetable tissue, such as pieces of epiderm, isolated sclerenchym-cellules, bundles of vessels, etc.-all of them looking as if they had been cleaned by some reagent, so that only the hardest " skeletal" parts were left. There are also to be seen the spiral threads, these being the only remains of spiral vessels, the thin walls of which probably have been digested. But I could not detect any parenchymatous cellules or other softer parts. I think, therefore that it may be admitted that the softer cellulose has been decomposed and digested, leaving only the more or less lignin-like substance. In Pseudochirus the small intestine is, as usual, thin-walled and villous but otherwise smooth. The walls of the ileum do not seem to show any increase in thickness. The ileo-caecal valve (v.i.c.) is well-developed, and from it extends as a transversal fold a caeco-colic valve (v.c.c.) and sphincter, as may be seen in the figure (text-fig. 5, p. 18). Close by, but on the colic side and also near the ileo-caecal valve a brownish glandular patch is seen, homologous with that described above and in a similar situation in Trichosurus. (It is PROC. ZOOL. Soc-1902, YOL. I. No. II. 2 |
