194 SUPPLEMENT 
mode of locomotion which caused M. Poli to extend, though 
evidently erroneously, the denomination of swbszlentia to all 
the acephala; for if most of the animals of the family of the 
conchs can leap in this manner, the submytilacea, the ar- 
cacea, &c. cannot, and appear in reality to crawl with their 
foot ; much less can a similar power belong to the species 
which have but a rudiment of this organ, or to those which 
do not possess it at all. 
The oscabriones move by crawling with their abdominal 
foot pretty much in the same way as do the patelle. As for 
the nematopods, no species belonging to them possesses the 
faculty of changing place altogether. The appendages of 
their caudiform abdomen can issue from the shell, and move 
in the water, but, as far as it appears, only to determine the 
current of that fluid into the interior of the mantle of the 
animal, and to seize the little animals that pass within reach. 
The composition of the mollusca is complete, that. is, it is 
formed of organs of digestion, of respiration, and of circula- 
tion. We shall first treat of those of digestion. 
The mouth is always anterior in the mollusca, although it 
be not constantly terminal or visible. It is sometimes alto- 
gether inferior, as in doris, onchidion, scyllza, oscabrio, &c. 
In fact, it does not appear that it is ever situated superiorly, 
Its form is extremely variable, depending on the disposition 
of the lips, which is different in the different groups. Thus 
in sepia and the neighbouring genera it is a sort of circular 
veil, sometimes double, pierced in its middle and fringed in 
its circumference. In the polybranches, cyclobranches, in- 
ferobranches, and even in many cervicobranches, they form 
a thick semicircular pad, in the inferior middle of which is 
the mouth, and which is prolonged sometimes on each side 
into a sort of appendage, which forms the labial tentaculum. 
In many species of doris, in tritonia, &c. the anterior edge of 
this labial pad is dilated, fringed, and forms a membranaceous 
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