t^^l

• XVlll INTEODUCTION.
this being accomplished by a sharp contraction of the pedal muscle,
the sliell and the remainder oi the animal are forced suddenly
forward.
Both Gastropods and Peleeypods, however, are not d^^pendent
on the foot only for voluntary movement, amoug the torriier
LiviiKea especially, and among the latter Pisidium are able to put
forth filaments of mucous by which they are enabled to ascend
and descend to and from the surface of the water and not only to
float on, but to skim along it, foot uppermost.
Tlie more or less artificial dispersal of mollusca is a subject
upon which, from time to time, a good deal has been written;
undoubtedly the larval Gasteropod forms are often swept many
miles by floods, frequently by this means being stranded in pools
and ponds isolated in normal times and to which it would
necessarily be impossible for them to obtain access were it not for
the agency of waterfowl which, there can be little doubt, do carry
certain species about in the plumage just above the legs, considerable
opportunity being given for the mollusca to ascend these
and become entangled in the feathers during the lengthy periods
in N-shicli tbe birds Tbtnain stationary in tiie watex watetiing ioT
their prey.
The Pelecypoda also are great travellers, but this is due to their
being carried far in their embryonic state by the hosts to which
they have attached themselves; once the parasitic larval stage is
over, they are seldom able, except through an exceptional accident,
to move far afield.
Prom the foregoing remarks, however, it must not by any means
be assumed that all the fluviatile species are widely distributed,
many instances occur of extremely localized forms, among which
may be mentioned as exam])les the Limncea mvoluta of Killarney
and some of the peculiar forms of Lake Tanganyika which do not
seem to have become diffused in the remainder of the Congo basin.
6. Economic Uses.—The purification of water is doubtless to
a certain extent performed by the mollusca in their capacity of
scavengers, in addition to wliich they also provide food for valuable
birds and fish; as a rule, however, the fluviatile species are
not in much request as food for man ; though in Guadeloupe,
Mauritius, China, Japan, and even in Prance, certain species of
freshwater bivalves are used for human consumption.
The chief services rendered to the human race are, however,
rather ornamental than actually useful; in the Unites States
certain species of Unionidoi are extensively fished and even cultivated
for their nacre which is used largely for button punching,
small kuife handles and other purposes to which mother-6'-pearl
is put, while as ai bi-product pearls themselves are doubtless
obtained ; indeed, a case has recently been placed on record of as
many as nine hundred and twelve being taken from a single
individual *, one hundred of which wore of marketable size, though
* INantilus, Boston, JIass.. xxv., p. 84.