282 Dr. H. Lacaze-Duthiers on the Float of the lanthine. 
every moment, in consequence of the folding of its margins 
downwards*. It is the moveable anterior portion that constructs 
the float; and this is effected as follows :— 
It is seen at first to become elongated in front, then to curve 
and become elevated, pass to the right and left, and embrace in 
its concavity the anterior extremity of the float, upon which it 
moulds itself. In its movements of elongation, this part of the 
foot often acquires the form of a small club, especially when it 
rises above the surface of the water. The position of the foot 
upon the anterior extremity of the float has been indicated by 
Mr. Adams. 
But it is especially necessary to follow the succession of the 
movements or manceuvres of the anterior part of the foot when 
it issues from the water and approaches the float. The foot is 
first of all seen to elongate itself, so as to issue from the water 
in a direction nearly opposite to that of the float, then the ani- 
mal lifts it up and causes it to project above the liquid. At this 
moment the organ presents the appearance of a cup at its ex- 
tremity ; it becomes hollowed into a canal by the approximation 
of its margins beneath, and slightly wrinkles its anterior por- 
tion. All these movements of course take place without inter- 
ruption, but their succession may be observed without difficulty. 
When the foot has issued from the water, the animal moves 
it backward, causing it to describe an arc, which removes it from 
the head and approximates it to the float; but at the same 
time the animal bends it in such a manner that the channel and 
the cup, which were turned upwards, become inferior. Then this 
extremity of the foot encloses beneath it a certain quantity of 
air, like an inverted glass or bell immersed under water....In 
this position the foot gradually approaches the top of the float, 
and it is then that we see it spread out and slide gently in all 
directions, as if it was gluing the surface of the float by creeping 
upon it. 
When this manceuvre, which Mr. Adams observed, without, I 
think, appreciating its purpose, has continued for a certain time, 
* We must apply a clear and precise meaning to the words “above” 
and “below,” in order to render the descriptions intelligible. The 
Tanthina, when swimming suspended from its float, is reversed, like a 
Limnea, which swims by gliding with its foot at the surface of the water. 
Hence, when we speak of the inferior surface of the foot, we mean of the 
foot in its natural position; and when, in the preceding paragraph, it was 
said that “the foot is hollowed into a canal beneath,” this relates to the 
position of the animal supposed to be erected and creeping upon the feot. 
For if we take the position of the animal beneath the float absolutely. this 
ought to be described as the superior surface. It will be remembered, 
therefore, that the words “‘ above” and “ below” relate not to the reversed 
animal, but to the animal supposed to be in the normal position of a 
Gasteropod. 
‘eos “ws = ee Sr “ee Pees & 
