ANDROGYNOUS ANIMALS. 423 
of laying. This is a case somewhat analogous to those 
quadrupeds which produce several young at a birth with 
one impregnation, differing, however, in the circumstance 
that the eggs are not all produced at the same time, al- 
though they are afterwards hatched by the same incubation. 
In the aphides, or plant-lice, as they are called, one im- 
pregnation not only renders fertile the eggs of the indivi- 
dual, but the animals produced from these, and the eggs 
of those again, unto the ninth generation *. 
III. Anprocynovus ANIMALS. 
The structure of the reproductive system in those ani- 
mals which have the sexes united in the same individual, 
exhibits two distinct modifications. In the first, impregna- 
tion can only be effected by the union of two individuals. 
In the second, the hermaphroditism is complete, and im- 
pregnation takes place without any assistance. 
1. In nearly all those androgynous animals, where there 
is sexual union, the testicle is single,—there is an external 
penis,—and the opening for the escape of the spermatic 
fluid is situated at its base. The ovarium is also single. 
The external openings of both organs are uniformly situat- 
ed on one side of the body. In some cases, they terminate 
in a common cavity, while, in others, the openings of the 
male and female organs are removed to some distance from 
each other. 
Impregnation, in the animals now under consideration, 
takes place internally, by the mutual application of the 
sexual organs of two individuals. The eggs, in a few spe- 
cies, are retained until hatched; but, in general, they are 
* «¢ Observations on the Aphides of Linnzus,” by Dr Wm. Ricuxarpson, 
Phil. Trans. 1771, p. 182—194. 
