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- 
glands 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 pair of collar-bones,or clavicles, and 
two blade-bones, or scapulz; 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 
