t^^l

XIV iNinoDtrcTiox.
chiefly owing to the Lack of properly preserved material fgtr
iuvestigatiou ; it may not, however, be out of place to here insert
a few binomic notes which concern the families generally, these
being grouped under the various headings as below :—
1. Breeding Habits.
2. Larval Stages.
3 Development from Post-Larval Stages.
4. Habitat and Mode of Life.
5. Movements, Locomotion, and Dispersal.
6. Economic Uses.
1. Breeding Habits.—Generally speaking the sexes in both the
freshwater Gastropoda and Pelecypoda are distinct, though here
and there examples of hermaphroditism are Jmown to exist, as,
for instance, in the case of certain species of Anodonta.
In the former case, the sexes can frequently be distinguished
by the form of the shell, that of the female being of a larger size
and generally more inflated than is that of the male. In the
Gastropoda this is especially to be remarked in the Viviparida;,
while in the Pelecypoda it is chiefly evident in the UnionidsE, the
female in this group being noticeably broader than the male.
In the fluviatile Gastropod families sexual contact usually takes
place, ova as a result being de])Osited in the majority of instances,
though in the case of the Tiaridse and Viviparidse, as the name of
the latter would imply, the young are produced alive.
In the Pelecypoda, however, union of the sexes does not take
place, the male at certain seasons in the year freely discharging
the spermatozoa into the water, these being introduced into the
inhalent siphons of the female by means of currents set up by
ciliary movements, fertilization taking place either in the oviduct
itself or else in specialized spaces of the mantle cavity.
The quantity of eggs or young produced by the different
families varies enormously, this, while attaining in the Pelecypoda
to thousands, and sometimes even to hundreds of thousands, as in
the case of certain species of Unio and Anodonta, falls in Planorhis
and Limncea to anything from twenty to a hundred, and in Ancylus
to such a small total as five or six only, while Vivipara and Tiara
average not more than about fifteen individuals at .a time.
In Tiara the embryos are developed in a marsiipium which is
formed by an infolding of the skin near the base of the right
tentacle, while in the TJnionidse, CyrenidsB, and some other
Pelecypoda development takes place in the spaces betweenlthe
folds of the gills where, in the Cyrenidas, special marsupia exist
for their reception'.
2. Larval stages.—In the fluviatile Gastropoda at birtli the
animal is generally more or less similar to that of the half-grown
or adult state, though the shell differs considerably; in the earlier
stages it is exogastric or coiled forward over the head of the
animal, but rapidly assumes the normal spiral of the adult. In