Dr. H. Lacaze-Duthiers on the Float of the lanthine. 281 
at the bottom of the water, although they were quite alive—that 
some of the more lively ones crept, although with difficulty, 
with their foot applied to the walls of the vessels, arrived at the 
surface, then turned themselves up, but most frequently without 
succeeding in reconstructing their float, and finally fell heavily 
to the bottom of the water. 
I never saw them swim, as so many Mollusca are seen to do, 
by alternately dilating and contracting the foot. Perhaps in 
the open sea things may proceed differently, but of this I can 
say nothing; everything seems to indicate that the shell and 
the animal are of a weight which does not allow them to swim 
without a float; and it must be added that the Lanthine which 
remained at the bottom of the water quickly died there. 
The unsuccessful efforts made by the animals either to return 
to the surface, or probably to reconstruct their float, gave me 
the idea of placing them in different conditions, and which, as it 
appeared to me, must be those sought by them. 
I first of all endeavoured to ascertain exactly the constitution 
of the frothy mass, and, like preceding writers, I soon found 
that there was no organic relation between it and the body, but 
that it was merely adherent to the foot, and consequently that 
the air which it contained, as it could not be the product of a 
secretion, must have been imprisoned or mechanically enclosed 
within the vesicles. The thing to be sought, therefore, was the 
means or mechanism by which the animal was able to introduce 
a bubble into each vesicle. 
The float is very regularly formed; the cells composing it are 
polyhedral in consequence of the mutual pressure which they 
exert upon each other, but they are always perfectly spherical 
in the part that remains free. This may be very well scen, for 
example, in all the vesicles of the circumference of the organ, 
upon the upper surface, and especially in the newly-formed 
cells. Moreover in the arrangement cf these vesicles there is 
a well-marked order: they form nearly straight lines running 
from one end of the mass to the other, and the greatest length 
of which is in a direction from before backward. 
By carefully observing the anterior extremity (that is to say, 
that nearest the head), one may exactly count the number and 
positively ascertain the volume, form, and relations of these 
terminal cells or vesicles. We may then trace and judge of 
what takes place when the animal is at work in restoring or 
increasing its float. 
The foot is very distinctly divided into two different parts: 
the posterior and larger one is flat, and it is this which furnishes 
the insertion for the float ; the other, or anterior one, is rounded 
in front and hollowed bencath by a canal which changes its form 
