ON MOLLUSCA. 245 
malies; such is that of the carychia, in which the true ten- 
tacula disappear by little and little, while all the other 
animals of the same family possess them in a very evident 
manner. 
The position of the eyes is also worthy of some considera- 
tion, but less perhaps than the tentacula. We find, in fact, 
some genera of the same family, or even some species of the 
same genera, which have eyes subpedunculate, and others 
which have them sessile. 
The form, the position of the principal organ of locomotion, 
that is to say, of the foot and the natatory appendages, give 
rise to considerations of still greater value in the classification 
of the malacozoaria; and as the characters derived from them 
are evidently exterior, it is not astonishing that they have 
been employed so often, and with much advantage. 
A better character perhaps in the relation of its importance, 
but unhappily more difficult to observe, which no doubt has 
prevented its employment, is that which may be drawn from 
the armature of the mouth, either at its orifice or in its in- 
terior, since it is in relation with the nature of the aliment. 
Another still preferable, because it agrees with the form of 
the shell in general pretty well, is derived from the position, 
the symmetrical or non-symmetrical form, and even from the 
structure of the organs of respiration. But unluckily, though 
these organs are most frequently pretty nearly exterior, a cer- 
tain degree of habit is necessary to enable us to employ this 
character with advantage. 
In fine, the part of the organization of the mollusca which 
appears as yet to present the character of the greatest value, 
is that which constitutes the apparatus of generation, com- 
posed of the two sexes in different individuals, or united in 
one individual, or, in fine, formed of a single female sex. 
Unhappily, again, this character is entirely anatomical, and 
consequently of difficult application in zoology. 
