Pvn H,i I'UitlS

6 BRACHIOPODA.
not been satisfactorily made out. Mr. Hancock supposes
that they maintain the vitality of the 'shell, and
that perhaps by their means any injury to it may be re-
paired. Some provision of this sort appears
to be neces-
sary, because the Brachiopods do not, to any great ex-
tent, thicken their shells by successive internal layers,
like bivalve Mollusca. The shells of the Brachiopocla
are never provided with an epidermis ; and this may also
account for their perforated structure. Many bivalve
shells, such as those of Astarte, from which the epidermis
has been accidentally removed, peel off, or become
eroded near the beaks to such an extent that, if new
layers were not continually being secreted from within,
the animals would be laid bare and exposed to untold
dangers. Sponges, Balani, Serpulce, and other extraneous
organisms are often seen covering or attached to
the shells of Terebratulce. The hinge in the articulated
kinds is so firmly interlocked, that it is impossible to
separate the valves without using a slight degree of force.
They appear to be in some measure sensible of light.
Lacaze-Duthiers gives an instance of Thecidia collapsing
suddenly when his shadow passed between them and the
sun. He succeeded in keeping these curious Brachio-
pods, in a lively and active state, for six weeks by merely
changing the water every day. The sexual nature of the
Brachiopoda is not quite determined. Dr. Gratiolet sug-
gests that the same individuals may become successively
male or female at different periods. Mr. Hancock con-
siders Lingula, at least, to be androgynous or monoecious ;
and he infers from analogy that both sexes are combined
also in the articulated Brachiopods. But Lacaze-Duthiers
has investigated this part of the subject more recently
and under circumstances more favourable than seem to
have fallen to the lot of any other writer. His elabo-