MOLLUSCA. 61 
destitute of a head; some of them have no locomotive 
power ; and, in others, the organ of motion is a fleshy foot, 
which the animal can protrude at pleasure. These circum-: 
stances point out the connection which subsists between 
the organs of the animal, and the external forms of the shell; 
a connection which, in every system, ought to be carefully 
attended to. 
It is somewhat difficult to point out, among the univalves, 
the true subordination, or relative importance of the cha- 
racters employed by conchologists in describing them. We 
have much to learn of their anatomy, and hence we cannot 
with certainty point out the relation of the parts of the shells 
to the organs which those parts protect. The form andstruc- 
ture of the mouth of the shell, however, may be expected to fur- 
nish characters of the first-rate importance, and have always 
attracted the notice of the student of testaceous bodies. The 
very shape of the animal, together with its ordinary habits, 
must necessarily depend, in a great measure, on the form of 
the mouth. 
_ In many genera, the mouth of the shell towards the base 
is produced, and terminates in a groove or beak. These 
univalyes are termed canaliculated, and are readily distin- 
guished from those whose mouth is enéive. The differences 
in the form of the shell in these two divisions is an index of 
equally remarkable differences in the form of the animals. 
The canaliculated shells contain animals possessed of an 
elongated tube for the purposes of respiration, and this canal 
is destined for its reception and protection when expanded. 
-The animals whose shells are destitute of this canal, are 
likewise destitute of this lengthened respiratory tube.  Cir- 
cumstances of this kind induce us to believe, that shells, 
agreeing in external form, in general, contain animals of a 
