4S8 DR. P. CHALMERS MITCHELL ON THE 



Sub-Order DUPLICIDENTATA. 



Family Ochotonidjs. OcJwtona rufescens. 

 In the Mouse-Hares the general pattern of the alimentary canal is similar to that 

 about to be described in the Rabbit. There is a well-marked duodenum. Meckel's 

 tract is composed of a large series of contorted minor loops at the circumference of a 

 nearly circular expanse of mesentery. The caecum is enormously long and capacious. It 

 is coiled tightly and its extremity forms a long vermiform appendage. Just at the point 

 where the small intestine enters the caecum (fig. 46, II, p. 517) there is a second caecum, 

 in the form of an elongated, slightly curved, hollow papilla, which appears to be, with- 

 out doubt, the vestigial representative of the second of an original pair of caeca. The 

 coiling of the caecum has involved the greater part of the first colic loop. The second 

 colic loop is lung and distinct, and the rectum is relatively long. I have seen only 

 preserved viscera of this genus, and I was unable to follow out the veins in detail : 

 they appeared to be similar in arrangement to those of the Rabbit. 



Family Lepokid.e. Lepus cuniculus (fig. 30). 



In the Rabbit the duodenum is a well-marked simple loop. Meckel's tract is very 

 long and is composed of a series of minor loops nearly equal in size, and a terminal 

 recurrent portion suspended at the margin of a long, oval, narrow-necked expanse of 

 mesentery. The caecum is very long, capacious, and ends, as is well known, in a finger- 

 shaped, narrow, thick-walled vermiform appendix. It is spirally twisted and the 

 twisting has involved a large portion of the end of Meckel's tract as well as the first 

 portion of the hind-gut. In the figure the latter is represented after having been 

 dissected off the spirally-coiled caecum. The second loop of the hind-gut lies folded 

 over the Meckelian mesentery and is also represented in the figure as dissected away. 

 The rectum is relatively short and straight. The anterior mesenteric vein curves round 

 the Meckelian mesentery, receiving tributaries from the duodenum and from the minor 

 loops of the tract. It is joined by a very large vein which branches over the coiled 

 caecum and the first loop and part of the second loop of the hind-gut. There is a short, 

 straight posterior mesenteric vein draining the rectum. 



In the embryo Rabbit the duodenal loop is relatively much longer, forming a narrow 

 simple loop. The minor loops of Meckel's tract are much shorter and less numerous. 

 The coiling of the caecum is much slighter, so that that organ appears as a long, com- 

 paratively simple diverticulum, which at its proximal end has just begun to be spirally 

 twisted. The first colic loop is nearly free from the caecum, and the second loop is 

 relatively longer. The appearance of the region in which the sacculus rotundus occurs 

 suggests that that organ is the remnant of a second caecum, the other member of an 

 original pair of caeca. In the Hare the second caecum appears in a much more definite 

 form (fig. 46, 1, p. 517). 



