ON MOLLUSCA. 229 
which, although contained within limits sufficiently bounded, 
are not the less very evident. But it sometimes happens that 
they depart considerably from the limit determined for a spe- 
cies, in that the turbinations of the spire become more apart, 
are elongated in a vertical direction, and are very far from 
touching, which gives to the shell the appearance of a ladder, 
and has given a denomination to the individuals of this ano- 
malous kind derived from this resemblance. An example of 
this may be found in some species of helix. 
But the most inexplicable monstrosity of shells, and even of 
the molluscous animals themselves, is that in which there is 
an inversion in the position of the viscera, and consequently 
in their termination, which, instead of taking place at the right 
side, takes place at the left. *The shell having followed this 
inversion, is then turbinated from right to left, and constitutes 
the variety to which the denomination of sznister has been 
given. There are some genera, in which this inversion is 
much more common, and even to the point of serving as 
a character. Such are physis and planorbis. In many other 
genera we find examples of it, but more rarely ; and finally, 
there are some in which it has not been observed, such as the 
porcelaines and cones. 
The intelligence of the mollusca, at first pretty evident in the 
higher species, such as the octopi, which employ stratagems 
to reach and seize their living prey, decreases very rapidly, and 
arrives at its minimum in those, all of whose movements are 
confined to the opening and closing the valves of their shell, 
as the oyster, and which receive their nutriment in the form of 
dissociated molecules, and already nearly in a fluid state ; 
the habits and mamners, therefore, of this class, do not afford 
much matter of interest, when compared with those of the 
other classes. 
The mollusca are found in habitations of all kinds. In fact, 
there are some which appear to live constantly under ground, 
