ON GASTEROPODA. 351 
water, where they swim in an inverted position, like the 
planorbes, the limnzz, and many other mollusca, with the 
assistance of their small foot. André Dupont says that the 
middle line of what he names the back, but which in fact is the 
belly, appeared like a leaf of silver, and was in a continual 
undulatory movement. ‘This little animal, little more than 
an inch in length, in consequence of its fine blue colour, 
silvery under the foot and at the extremity of the digi- 
tations, and especially from its form, appears extremely 
elegant when it is swimming in calm weather at the surface 
of the sea. 
The little order of the INFEROBRANCHIA we must entirely 
pass over here, as nothing whatever is known concerning 
their habits or manners. 
Of the first two genera of the order TECTIBRANCHIA, we 
know nothing, except what regards structure. The genus 
APLYSIA, by a typographical error, in the second edition of 
Linneus, was called Laplysia, which misnomer, ridiculously 
enough, has been preserved. Pliny and Dioscorides speak 
of a species of this genus under the name of sea-hare, and 
depict it as a venomous animal, which not only should not be 
touched, but not even looked at. Following them, Rondelet 
has spoken in the same manner. 
The Aplysia are supposed to possess the property of 
causing the hairs to fall from those parts of the body to which 
they are applied, and of causing strangury to those who are 
so foolish as to swallow a little of the sanies which flows from 
their bodies. But M. Cuvier has proved this opinion to be 
erroneous, at least as far as the first of these properties is 
concerned. But they exhale an odour so nauseous and fetid, 
that one would be more disposed to avoid than approach 
them. 
The best known of the species resembles, when in a state of 
repose, a mass of unformed flesh. When it is in motion its 
