MOLLUSCA ye 
reached the extremity, launched itself off from it ; 
after moving about with a sort of swimming or 
rolling motion in a horizontal direction for some 
time it lowered itself gradually, and in effecting 
this, the long flexible leaf of the Vallisnerta was 
bent with an undulating motion, corresponding 
exactly with every movement of the snail, clearly 
showing that it had a firm attachment to the ex- 
tremity of the leaf. On another occasion a LZ. 
glutinosus gradually rose from the surface of a piece 
of submerged rock, and when at the distance of 
about three or four inches from it, stayed its pro- 
gress, floating about in a circumscribed horizontal 
direction for some time; at last it rose suddenly 
and rapidly to the surface, evidently from the 
rupture of its thread of attachment. The most 
convincing proof, however, of this fact, that I can, 
perhaps, adduce, and one that I have often repeated 
with all the before-mentioned Limnez, is that when 
the snail has been some inches distant from the 
supposed point of attachment, a rod or stick has 
been carefully introduced, and slowly drawn on 
one side between them in a horizontal direction, 
and by this means the snail can be made to un- 
dulate to and fro, obeying exactly the movement 
of the rod: this requires to be done very gently, 
as, if too much force is used, the web is broken, 
and the snail rises rapidly to the surface.’’ * 
The wide expanse of ocean from the equator to 
the poles is tenanted by a class of swimmers, 
small, indeed, in the number of its species, but 
countless in the hosts of individuals of which they 
are composed; the Pteropoda. Some of these in- 
habit shells, which for delicacy and transparency, 
* Annals of Natural History. October, 1852. 
