﻿MONOTREMES. 227 



that there is, as in Birds and Reptiles, but a single aperture at 

 the hinder extremity of the body from which are discharged the 

 whole of its waste-products, together with the reproductive 

 elements ; the oviducts opening separately into the extremity 

 of this passage, which is termed the cloaca. Reproduction is 

 effected by means of eggs, which are laid and hatched by the 

 female parent ; while, after the extrusion from the egg, the young 

 are nourished by milk secreted by special glands situated 

 within a temporary pouch, into which the head of the young 

 animal is inserted and retained. The ducts from these milk- 

 glands instead of opening into teats, discharge merely upon the 

 porous skin of the inside of the pouch by means of numerous 

 apertures ; the milk being forced into the mouths of the young 

 by the contraction of special muscles. The milk-glands are of 

 simpler structure than those of other Mammals, and, according 

 to Gegenbaur's researches, correspond to the sweat-glands of 

 the latter, and not to the milk-glands ; so that these milk- 

 giands of the Monotremes do not correspond homologically 

 with those of other members of the class. 



The skeleton differs from that of the higher Mammals, and 

 thereby corresponds with that of certain lower Vertebrates, in 

 regard to the structure of the shoulder-girdle, or that portion 

 which serves for the support and attachment of the bones of the 

 fore limb. In nearly all the higher Mammals the shoulder- 

 girdle comprises merely a pa'r of collar-bones,or clavicles, and 

 two blade-bones, or scapulae ; each of the latter having a 

 projection on its lower extremity termed the coracoid process, 

 — so-called on account of a fancied resemblance in man to the 

 beak of a Raven. In the Monotremes, on the other hand, there 

 is an additional T-shaped unpaired bone lying on the collar- 

 bones and breast-bone, known as the interclavicle ; this bone 

 occurs in no other Mammals, but is well-developed in 

 Lizards and many extinct Reptiles. In place of the coracoid 



Q 2 



