INTESTINAL TEACT OF MAMMALS. 525 



tract, moreover, is never arranged in a series of definite loops, the number, position, 

 and shape of which are of systematic importance, as is the case in Birds. Thirdly, a 

 narrow portion of the originally nearly straight hind-gut, distad of the caeca, a region 

 which in Birds practically invariably remains primitively short, may grow out to fqrm 

 long colic loops. The interconnection that takes place in Mammals is between one of 

 these colic loops, a portion of the hind-gut distad of the caeca, and a portion of 

 Meckel's tract not limited to the duodenum. Moreover, the interconnection is 

 different in its nature in the two groups. In Birds the supra-duodenal loop is applied 

 directly, and with an accuracy that becomes more perfect in the higher members of 

 each series, actually to the fore-gut itself, as it forms the duodenal loop. In Mammals 

 the colic loop is applied to the mesentery that suspends Meckel's tract along the bases 

 of the minor loops of that tract, so that the colic loop is applied to an area of Meckel's 

 tract very much shorter than itself, following the sweep of the mesentery, but not 

 imitating the actual loopings of the small intestines. However, the interconnections 

 of the mesentery and of the blood-vessels may be elaborated in Mammals as in Birds. 

 Thus, out of materials that are not homologous, a very remarkable and apparently 

 adaptive relation has been produced in the two groups. 



I may now give a systematic summary of the extent to which this interconnection 

 of the gut occurs. 



Monotremata. Slight, and with the duodenal region in Ornithorhynchus ; practically absent 

 in Echidna. 



Marsupialia. Practically absent throughout. 



Tubulidentata. No information. 



Pholidota. Absent. 



Xenarthra. Absent. 



Hyracoidea. Absent. 



Sirenia. Absent. 



Proboscidea. Absent. 



Cetacea. Absent. 



Artiodactyla. 



Non-Rominantia. Absent. 



Traguloidea. A large colic loop, distad of the spiral coil, is folded against the mesentery of 

 Meeker's tract, but the connection is not so intimate as in the Pecora. 



Tylopoda. A colic loop of moderate length folded over against the Meckelian mesentery, 

 but the interconnection not so definite as in the Traguloidea, and not nearly so definite 

 as in the Pecora. 



Pecora. In all these the interconnection is extremely definite, being much more elaborate 

 than in any other Mammalian group. The very large colic loop is folded so closely 

 against the Meckelian mesentery, following the latter along the curved line of origin of 

 the minor loops of Meckel's tract, and the mesenterial fusion and interconnection of 

 blood-vessels is so complete, that an elaborate dissection is required to separate the 

 two regions of the gut. Anyone who observes this in a number of Pecora must be 

 VOL. XVII. — PART v. No. 12. — lkr< mix r, 1905. 4 A 



