190 SUPPLEMENT 
There is a third method, more singular, and which is to be 
met with in the first species of the type and in the last. It is 
executed by the contraction of the envelope, which thus ex- 
pels the fluid with which it has been filled in its dilatation, 
from which results a movement of translation, often very 
quick. The sepia, the calamary, or loligo, and the biphore, 
move in this manner. 
Some mollusca sail upon the surface of the waters, impelled 
by the current, or by the wind, some by the assistance of a sort 
of hydrostatic bladder, as the janthinz and others, by unfurl- 
ing a sort of sail, formed by the salient edge of the mantle, or 
by some widened appendage, while they row with others, as is 
related of the argonaute. In the first instance, it appears 
that the animal is always at the surface of the water, for it 
cannot make the bladder re-enter, which is subcartilaginous. 
In the second, the octopus can, it is said, at will, develope its 
sail and oars, or refold them within the shell, which serves it 
as a boat, and then sink more or less deeply. But after all, 
may not some degree of doubt yet attach to the truth of this 
ingenious manceuvre ? 
Perhaps it is the octopi only which perform a sort of walk, 
by means of the long appendages which crown their head ; but 
then they have the mouth below and the trunk uppermost. It 
appears also that they can roll over and over at the bottom of 
the sea with great velocity, and without fixing themselves by 
their tentacula. 
Some species of auricule, and even the terrestrial cyclosto- 
mata, also make sorts of steps, by taking a resting point on 
the anterior part of the foot, or on the advanced muzzle, and 
approximating to it the posterior part or centre foot all at once. 
A much greater number crawl on the surface of the ground, 
either on land or in the waters, by means of the foot or mus- 
cular disk with which their belly is provided. But this sort 
