Dr. H. Lacaze-Duthiers on the Float of the lanthine. 283 
the foot withdraws, quite gently, under water, to remain there if 
its operations cease, or to move forward again and recommence 
if its work is to be continued. 
When we have counted the number of vesicles at the extremity 
of the float, and thoroughly observed their arrangement before 
the manceuvres just indicated, we see, when they have ceased, 
that another cell has been glued on in front of those which we 
had ascertained to be the furthest towards the mouth. This 
first fact proves incontestably that the growth of the float takes 
place longitudinally and at its anterior extremity. It proves 
also that it is the foot which manceuvres in such a manner as to 
add the new vesicles towards this extremity; and the process 
of growth is, no doubt, as follows :—The foot, first of all curled 
up into a cup, had, when applied to the float, a certain thickness 
of air between it and the latter; by secreting a layer of mucus 
and then spreading out, it must necessarily join this viscous 
layer to the rest of the float, and thus keep the air-bubble im- 
prisoned. 
We may form an idea of what takes place when we observe a 
garden-snail or slug creeping upon a body covered with dust : 
we often find, beneath the train of mucus left behind it by the 
animal, a bubble of air which is imprisoned between the un- 
moistened surface of the body and the lamella secreted by the 
foot. Here we have something produced, mechanically speak- 
ing, perfectly similar; but in the former case it is accomplished 
by design and for a particular purpose. 
The mucosity is evidently insoluble in water; and as it dries 
in the emergent portion, it acquires a certain consistence, which 
has led to the supposition that it was cartilaginous. 
If we admit the process of the formation of the float to be 
as just described (and it seems difficult to me not to regard it as 
true, since at each new movement of the foot we count a fresh 
bubble added), we may.easily explain the dissidences of authors, 
their opinions, and most of the facts which they relate. Thus 
we understand why the animal, when once at the bottom of the 
water, is incapable of forming a new float. Most probably the 
Janthine which have lost their float are fated to die, unless they 
be carried to the surface by some cause which I shall not attempt 
to imagine. 
* * * * * * 
To verify the notion which has just been put forward, it was 
necessary that experiments, varied in several ways, should fur- 
nish its confirmation. 
Having taken an Janthina upon a small iron hook and sus- 
pended it, not out of the water, but at about that depth below 
the surface which it occupies when it swims freely suspended 
