96 
Sanders^ 07i certain Pulmo gasteropoda. 
the right tentacle. During copulation it is everted, so as 
to bring the opening of the vagina close to the orifice. 
The penis is composed of two parts, the flagellum and the 
penis proper. One is at a loss to discover of what particular 
use the flagellum is, as it is not everted during coition, 
and does not appear to be glandular ; but it must be supposed 
to have some use in the economy of these animals, as they 
are at present victors in the "struggle for life,^'' and are 
therefore not very likely to be found weighted by any super- 
fluous organs. 
From the entrance of the vas deferens into the penis there 
run four folds of mucous membrane, which terminate in a 
sort of glans, which is contained in a loose muscular bag — 
the prepuce; this opens into the vestibule, close to its ex- 
ternal orifice, having passed beneath the retractor muscle of 
the right tentacle. This arrangement appears to bear some 
distant relation to what occurs in P. corneus, for a very little 
more would separate the male opening from that of the 
female; and the penis passing beneath the retractor muscle 
represents faintly the vas deferens of the Planorbis passing 
through the walls of the body. 
The facts which I have thus endeavoured to describe not 
only confirm the opinion of those writers who, notwithstand- 
ing the improbability of the idea, yet persisted in maintaining 
that one and the same gland, at the same period of time, se- 
creted both zoosperms and ova, but also show that the former 
— in these Invertebrata, at least — are developed in a contrary 
manner to that which takes place in Mammalia, in the latter 
being formed in the interior of cells, the "vesicles of evolu- 
tion in the former being those vesicles of evolution them- 
selves, simply altered in shape and attenuated. I may add 
that I cannot confirm the statement of H. Meckel, that each 
follicle is double, and that there is a double duct. The 
structure which he supposed to be a duct turns out to be a 
nerve, and the follicles of the dichogamic gland have always 
appeared to me to be single, the various contents not being 
separated from each other by any membrane, however thin. 
In conclusion, I wish to explain that I have adopted the 
term " dichogamic^^ in this paper in consequence of a sug- 
gestion I found in G. H. Lewes's ' Life of Aristotle,' in which 
he points out the absurdity of denominating by the same 
word the abnormal hermaphroditism of arrested development 
and the normal occurrence of bisexualism as in the present 
instance. He coined the above word, which I have accord- 
ingly adopted. 
