VIVIPAROUS ANIMALS. 415 
the loss of teeth, by adapting his food to the altered state 
of the masticating organs, and thus outlive the period at 
which the life of the other species is doomed to cease. 
Having thus given a general view of the sexual organs 
of viviparous animals, and the manner in which these exer- 
cise their functions, we are next led to consider a second 
class of animals with distinct sexes, in which, however, the 
ovum is not nourished by the circulating system of the 
mother. 
II. Cvirparous ANIMALS. 
Amone the viviparous animals, the reproductive organs 
present many points of resemblance, and appear to be con- 
structed according to acommon model. It is otherwise 
with the sexual organs of the oviparous tribes, which we 
are now to consider. They exhibit such remarkable diffe- 
rences in the form and structure of all thew organs, and 
occupy so many different situations, that it is impossible to 
collect them into natural groups, or assign to them charac- 
ters which they have in common. It will be expedient, 
therefore, in this place, to avoid all minuteness of detail, 
and to take notice of the peculiar modifications of the sex- 
ual organs exhibited by particular tribes, when we come to 
treat of these in the general classification. 
1. Male organs.—In the essential parts of the male or-. 
gans of oviparous animals, few modifications of any con- 
sequence present themselves. The testes are always con- 
cealed in the abdomen. They are in general two in num- 
ber, and obviously distinct from each other; but in some 
cases, among the mollusca, and annulosa, they appear to 
. be collected in a single group. The spermatic duct is 
either double or single, according to the structure of the 
organ from which it proceeds ; and at its external termima- 
tion in birds, for example, either opens into a tubular penis, 
