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. OVIPAROUS ANIMALS. 



AMONG the viviparous animals, the reproductive organs 

 present many points of resemblance, and appear to be con- 

 structed according to a common model. It is otherwise 

 with the sexual organs of the oviparous tribes, which we 

 are now to consider. They exhibit such remarkable diffe- 

 rences iu the form and structure of all their 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 termina- 

 tion in birds, for example, either opens into a tubular penis, 



