516 DR. P. -CHALMERS MITCHELL ON THE 



be taken as about sixty inches, the caeca open into the gut between three and .four 

 inches from the cloaca. In the human being, where the length of the whole tract from 

 the stomach to the anus may be taken as being about twenty-six feet, the caecum is 

 placed at a distance of about live to six feet from the anus, whilst in many Mammals, 

 as, for instance, a Rabbit or Sheep, the caecum lies very much nearer the middle of the 

 length of the gut. In a study of the intestinal tract of Birds (15) I was able to show 

 that in some Birds, e. g. the Struthious Birds, the hind-gut was relatively very much 

 longer than in the majority of Birds, with the result that the caecum in such forms lies 

 much more nearly in the common mammalian position — in the Ostrich, for instance, 

 almost midway in the course of the gut. It may be that in the Ostrich the hind-gut has 

 increased in length in the history of the transformation of a flying bird into a large 

 terrestrial form, but, on the other hand, it is equally clear that in the marjority of birds, 

 and especially in the Passeres, there has been a phylogenetic shortening of the hind-gut 

 with a consequent approximation of the caeca to the cloaca. On the other hand, if one 

 looks through the figures 1 to 45 which illustrate this memoir, it will be seen that whilst 

 in the majority of the mammalian types the caecum is nearer the posterior end of the 

 intestinal tract than in the case of the majority of Birds, the relative position varies, 

 and in many forms, especially Carnivora, it approaches the avian position. The morpho- 

 logical position, in fact, varies very little ; the apparent position varies in proportion to 

 the extent to which the hind-gut has departed from its primitive straight condition to 

 be differentiated into a more or less complex series of loops. In the vast majority of 

 Birds the hind-gut has apparently become shorter instead of lengthening ; in a very 

 considerable proportion of Mammals it has been lengthened. 



Occurrence of Paired Cceca. 

 In the vast majority of Birds the caeca form a pair of ventro-lateral outgrowths from 

 the wall of the gut. In certain cases, however, e. g. the Ardeidae normally, in Columbidae, 

 Vulturidae, and Plotus as individual variations, one of the pair may be completely absent. 

 In Mammals, on the other hand, the normal state of affairs is that one caecum only is 

 developed. The exceptions are, however, more numerous than is generally supposed. 

 N Amongst the Edentata the existence of a pair of symmetrical caeca is well known. I 

 have found them myself in Tatusia keppleri (fig. 12, p. 458), Dasypus villosus (fig. 13, 

 p. 459), Dasypus minutus, and Xenurus imicinctus. Rapp (23) has recorded their presence 

 in Myrmecophaga didactyla and Dasypus sexcinctus, comparing them with the similar 

 organs in Birds. Meckel (14), Owen (19), and Flower (5) have stated that they exist 

 in Oyclothurus didactylus, whilst Flower has figured them in the last-named genus and 

 in Dasypus sexcinctus. As I have already described and figured (fig. 14, p. 461), a 

 symmetrical pair is present in Hyrax, in the paired caeca being obviously comparable 

 with the normal caecum of Mammals in position. Finally, paired caeca occur in Manatus 

 (fig. 15, p. 464). 



