INTESTINAL TRACT OF MAMMALS. 463 



indeed, Meckel had originally described. Distad of the unpaired caecum, the less 

 elaborate thickening of the mucous membrane left a wider central cavity. The 

 longitudinal folds were much more regularly arranged than in the anterior portion, 

 and the villi were less numerous and much shorter, in considerable portions being 

 practically absent. What I take to be the colon proper, however, the region distad of 

 the paired caeca, shows a condition which I have not found described by any of my 

 predecessors in this field. The calibre of the central cavity is relatively much greater, 

 and although, as George stated, the internal surface is relatively smooth, there are 

 present numerous, but widely scattered, extremely long papillae which extend far into 

 the cavity of the gut, and in the sections frequently lay doubled up. 



Notwithstanding the very large amount of work that has been published on the 

 alimentary canal, I am disposed to think, and examination of Oppel's (16) careful 

 compilation of the literature confirms me, that we have not sufficient systematic 

 information as to the characters of the lining membrane of different regions of the 

 mammalian alimentary canal, to draw morphological conclusions from the evidence of 

 particular cases. In the case of Hyrax, there is a gradual change from the duodenum 

 towards the paired caeca, the change consisting in a reduction of the complexity of the 

 mucous membrane and an increasing preponderance of the glandular element over 

 the villi, whilst there is an abrupt and greater change behind the paired caeca, of which 

 the most marked feature is the sudden appearance of the very long scattered papillae. 

 So far the microscopical structure, if it does not give active support, at least is not 

 opposed to my view that the paired caeca are the representatives of the normal 

 mammalian caecum. I have no idea if any significance from the systematic point of 

 view is to be attached to the presence of the very long villi in the colon ; there is 

 practically no recorded information on this subject. Of my own knowledge I can say 

 that I have noticed the absence of colic villi in a good many Primates and Carnivora 

 and have not found them in these groups, whereas I have frequently found them in 

 Ungulates and Rodents. 



The general pattern of the intestinal tract in Hyrax, however, suggests no affinity 

 with the patterns exhibited by Eodents and Ungulates. The simple duodenum, the 

 nearly circular Meckel's tract, and the hind-gut divided into a simple colon and rectum 

 merely conform to the fundamental mammalian plan. The presence of the paired 

 caeca, on my view that paired caeca are a primitive mammalian feature, does not help 

 us with the placing of Hyrax. The most striking resemblances are resemblances with, 

 on the one hand, the patterns displayed by the Xenarthra among the Edentates, and, 

 on the other hand, with the Manatee among the Sirenia. But it must be remembered 

 that the common possession of a primitive simplicity is no guide to affinity. Otherwise, 

 it might be said at once that, in the character of its intestinal tract, Hyrax stands 

 nearest to the Edentates, Xenarthra, Marsupialia, and Sirenia. 



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