ON MOLLUSCA. 189 
part of the body is occupied by a sort of foot very analogous 
to that of the patella, phyllidii, &c., while the back, in its 
conchyliferous part, presents as many double pairs of oblique 
muscles, one at the right and the other at the left, as there 
are testaceous pieces. 
The faculty in the molluscum of changing its relations with 
external bodies being generally in a direct proportion with 
the quantity of the sensibility, it is evident that the locomo- 
tion of the malacozoaria cannot be very active, must be con- 
fined in its range, and often does not exist at all. 
The brachiocephala (cephalopods), being those mollusca 
whose sensorial faculties are most extended, are also those 
which move with the greatest quickness, and in all directions. 
The acephala, and especially the last of them, such as the 
ascidiz, are exactly in the opposite extreme, and in fact they 
live fixed upon immerged bodies. 
We remark, however, among the mollusca, many kinds of 
locomotion : a certain number swim by the aid of fins, or sorts 
of appendages, in pairs, such as the calamary, the sepia, the 
pteropods in general, and many monopleurobranchia, pretty 
nearly in the same manner, as do the fish with their pectoral 
fins. These organs even enable them sometimes to issue from 
the water and shoot into the air, to different degrees of dis- 
tance. This is a certain fact with respect to the calamary. 
The same may be said of certain species of bivalves, which 
thus employ the valves of their shell as a sort of wings, with 
which they take their point of support upon the water. 
Another sort of motion is that which is performed by an 
odd middle fin, or by a very compressed foot, and consequently 
by alternate movements to right and left, as may be seen in 
the firole and carinarie.. But in this case motion appears 
never to take place but in a reversed situation ; that is to say, 
the back being under and the belly uppermost. 
