IXTESTIXAL TRACT OF MAMMALS. 247 



As it is difficult to suppose that the different types of orgajis in 

 higher Mammals have arisen separately from the corresponding 

 types in Marsupials, we are faced with the possibility that 

 organisms may have inherited the possibility of displaying 

 definite variations tha,t have not actually appealed in their 

 ancestral history, a possibility which, as Arthur Willey hns 

 shown (Willey, 1911), has not yet been sufficiently^ considered in 

 systematic zoology. 



1 have also to note that the characters of the gnt-patterns 

 aflbrd no ground for grouping together the so-called Edentates. 



It is interesting to note that amongst other primitive chai'acters 

 this first gi'oup of mammals contain abundant relics of what I 

 take to be the jorimitive paired condition of the cteca. 



The Monotremes have not moved far from the primitive type, 

 but in. a definite direction. The duodenum is distinct ; the 

 ceecum is single but degenerate, and is placed very close to the 

 apex of the pendant loop, the distal limb of which displays a 

 compound ansa coli dextra, and the rectal poi'tion is relatively 

 short and straight. 



The Pholidota, although not far removed from the primitive 

 type, have lost the cfecum, and have a longitudinally striated 

 hind-gut Avhich is unique. 



The Hyracoidea, Sireuia, and Proboscidea have not moved far 

 from the common type, and I realize that their association may 

 depend very largely on their retention of primitive characteis. 

 In all, the primitive mesentery is very complete and continuous, 

 and although the hind-gut is long, there are no specialized loops. 

 The unpaired csecum of Hyracoidea, if I am correct in refusing 

 to identify it with the unpaired cjecum of other mammals, is a 

 peculiarity unique among mammals ; but apart from that, and 

 taking the paii'ed caeca as the true representatives of the noi'mal 

 structure, there is a very close resemblance between the pattern 

 of Hyracoidea and that of the Manatee. The chief difference is 

 the further increase of length of the hind-gut, distad of the 

 pendant loop, in the Manatee. The Dugong is said to have a 

 gut-pattern of the .same type as that of the Manatee, except 

 that the csecum is single, and such a pattern leads directly to 

 that of the Elephants, in which the cfecum is unpaired and the 

 hind-gut thrown into long irregular minor loops, so that it 

 appears to be almost as long as the fore-gut. There is no 

 trace of any of the peculiarities of pattern found amongst the 

 true Ungulata, 



In considering the Cetacea, the first feature of importance is 

 that the Odontocete pattern is most easily explained as a 

 derivative by reduction of a pattern such as is found in the 

 Mystacoceti. The Mystacocete pattern, although peculiar, is 

 not very far removed from the primitive type, but the great 

 elongation of Meckel's tract with the cfecum at its apex, the long, 

 nearly straight, recurrent limb of the pendant loop, and the long 

 but nearly straight rectum, make up a diveigence from the 



