ON MOLLUSCA. 219 
tubercle, as doris, tritonia, &c. Finally, in all the pulmo- 
branch species these terminations are made in a sort of com- 
mon vestibule at the root of the right tentaculum, so that in 
a state of inaction but a single orifice is seen at the exterior ; 
but in the act of coupling the vestibulary pouch is reversed, 
and the two terminations become apparent. 
The third disposition of the genital apparatus of the mol- 
lusca constitutes the division or isolation of each sex ina 
distinct individual, whence result females and males. As for 
the rest, each apparatus is pretty nearly conformed according 
to the preceding disposition. We find, however, perhaps 
more frequently in the female sex, the swelling of the second 
oviduct performing the office of matrix, and in the male sex 
we remark that the vesicule seminales are sometimes re- 
placed by a single swelling situated towards the end of the 
deferential canal. Finally, another difference is that the 
excitatory organ, when it exists, never appears to be com- 
pletely retractile, but only contractile, so that it is always 
more or less visible at the right and anterior side of the 
animal, sometimes being recurved into the branchial cavity. 
With respect to the products of the organs of generation, 
that of the male sex, when it exists, appears to be always a 
fluid of a viscous white, but in general it is little known. We 
are not even acquainted positively with the mode in which 
it is ejected, whether at once or by little and little, nor into 
what part of the female organ it penetrates. ‘The product of 
the female sex is much more known, and always constitutes 
a true egg, composed of envelopes of a vitelline mass, and 
a germ placed upon this mass, which no doubt makes part 
of it. 
The form of the eggs of the mollusca does not fail to ex- 
hibit a good number of varieties ; sometimes being spherical, 
as in those of the limaces; sometimes ovaliform, as in a great 
