MOLLUSCA. OT 
ferring the animal of the mya to the genus ascidia, we shall 
only mention, that the former has a foot, and possesses a 
locomotive power; the latter has no foot, remains immove- 
ably attached for life upon the substance to which it at first 
adhered, and depends on the accidental bounty of the waves 
for all its nourishment. 
The animals of nearly all the univalves are represented 
as belonging to the genus limax. But, with the exception 
of the restricted genus helix and bulimus, the animals of the 
univalves are all generically different from the limax. Their 
tentacula are generally two in number, with the eyes at the 
base ; while the tentacula are four in the limax, with the 
eyes at the tips of the two longest. These examples’ will 
suffice te establish a truth so palpably obvious. 
The principal objection against this system of employing 
the shell, to the exclusion of the animal, arises from the 
fact, that nature has not drawn a line of distinction between 
the mollusca and the testacea. Thus, many of the vermes 
mollusea of Linnzeus include shells in their bodies, as the 
aplysia ; and many of his vermes testacea likewise are soft 
on the outside, the shell being inclosed by the integuments, 
as the helix laevigata, now constituting the genus VELUTINA. 
We have another objection to this artificial system, and 
one which we consider of great force ; that, wherever it 
prevails, the form and habits of the animal are overlooked. 
How fully is this truth illustrated in the works of the testa- 
ceologists of this country! We might examine all their 
writings, from the Pinar of Merret to the Descriptive Ca- 
talogue of Maton and Raket, including the works of Pen- 
nant, Berkenhout, Da Costa, and Donovan, and learn little 
more, besides the habitat, than that to every shell there is 
an animal attached. Of this charge the names of Lister 
