INTESTINAL TK.ACT OF MAMMALS. 209' 



paired. In their structure, morphological position on the gut,, 

 position as seen when the abdominal cavity is opened, and 

 attachments, they correspond with the normal mammalian 

 caecum. The accessory caecum of Hyracoidea differs from the 

 normal mammalian caecum in structure, morphological position on 

 the gut, position in the undisturbed body-cavity, and attach- 

 ments. The attempt, based on minute details of structure, to- 

 identify the unpaired ca?cum of Hyracoids with the unpaired 

 caecum of a Perissodactyle such as the Rhinoceros, makes the 

 pi-esence of paired caeca still more inexplicable. I adhere, there- 

 fore, to my identification of the paired caeca of Hyracoidea with 

 the normal mammalian caecum. Owen (Owen, 1832) definitely 

 compared the paired caeca of Eyrax with the paired caeca of 

 Edentates and of birds, and the unpaired caecum with the 

 " additional single caecum, anterior to these, found only in a few 

 species (of Birds)." This appears to be the most reasonable 

 interpretation of the facts. I am unaware of any reason for 

 ]-efusing to identify the paired caeca of Edentates (and of the 

 Manatee) with the normal mammalian structure, and I have 

 shown good reason for identifying the normal caeca of birds with 

 the mammalian caecum or caeca. I have shown (Mitchell, 1901) 

 that what Owen calls the "anterior caecum, found only in a few 

 species," which, of course, is the remnant of the yolk-sac, is of 

 frequent occurrence in advilt birds, that its constant presence is a 

 character of many groups, and that in certain cases (Mitchell,. 

 1903) it is transformed from a vestigial structure to a well- 

 marked glandular organ. The corresponding structure in mam- 

 mals, known as Meckel's diverticulum, is a rare abnormality,, 

 but has been recorded as occurring in just over 2 per cent, of 

 human bodies. It is a diverticulum of the small intestines 

 lying almost exactly in the region where the unpaired caecum of 

 Hyrax is found. If this identification be correct, the unpaired 

 caecum, obviously functional in the adult Hyracoids, has acquired 

 an importance that is unknown in any other group ; but this 

 is a supposition less difficult than the view that the Hyracoids 

 display a loop of the gut identical with that of Perissodactyles 

 generally, a caecum corresponding in minute detail with the 

 caecum of the lihinoceros and paired caeca peculiar to them and 

 the Edentates. 



Examination of the intestinal tract of D. clorsalis, and con- 

 sideration of the points raised by Dr. Beddard, therefore, confirm 

 the view I stated formerly (Mitchell, 1905, p. 463). The general 

 pattern of the intestinal tract of the Hyracoids suggests nO' 

 affinity with the patterns exhibited by Rodents and Ungulates. 

 The simple duodenum, the nearly circular Meckel's tract, and 

 the hind-gut* divided into a simple colon and rectum merely 



* It is, of course, plain that by "hind-gut" I imply the region distad of the 

 paired caeca, as I reckon the part of the gut between these and the unpaired cascum 

 as part of the small intestine. When Beddard (1908, p. 583) stated that my de- 

 scription of the hind-gut was " incorrect " he was merely restating his belief that the 

 unpaired caecum was the true caecum, and that all the gut distad of this, including 

 what he took to be a paracsecal loop and the paired caeca, was hind-gut. 



Proc. Zool. Soc— 1916, No. XIY. 14 



