ON ACEPHALA. 411 
of the tubes of their mantle, they let in, as far as the mouth, 
and traversing the branchial cavity, the fluid which is to serve 
at once for the purposes of nutrition and respiration, while, 
through the other come the excrements, and the product of 
generation. ‘This double movement is produced by the action 
of certain organs, to which the name of palettes, or palmule, 
has been given. ‘They vary in form in each species, but are 
always similar to each other, and symmetrically placed, one 
at each side of the extremity of the mantle. In the common 
teredo, each is composed of a calcareous piece in the midst of 
a membranaceous part. These palettes are opposed to each 
other at their internal face, and, by being applied together, 
can close the orifice of the hole which the animal inhabits, 
after it has withdrawn the tubes of its mantle. These parts 
therefore serve as kinds of opercula, but they cannot be better 
compared than to the antenne of certain male insects. They 
are, in fact, composed of a considerable number of calcareous 
articulations. ‘The hole inhabited by the teredo commences 
with an orifice extremely small, which is often even difficult 
to perceive. Its beginning is always a little horizontal or 
oblique, but beyond that point it curves, and becomes more or 
less vertical or straight. The nature of the wood has a great 
influence on the regularity, and the flexions of the canal, which 
is hollowed in its interior. The neighbourhood of other 
teredines exercises a still more manifest effect upon it. In 
fact, they seem to endeavour to avoid each other, which some- 
times produces very considerable flexions. The depth of the 
holes is necessarily proportioned to the size of the teredo, and 
to the duration of its life, which appears to be rather short 
and rapid. But by what process does this animal, which, 
very probably, is ovo-viviparous, and rejects the product of 
generation, already provided with the shell, thus excavate 
the hardest woods, and that in so very prompt a manner? 
The form of the shell, so well adapted for boring and filing, 
