20 MOLLUSCA. 
A species of Certthium, found at the mouths of 
rivers in tropical countries, has the silkworm-like 
habit of spinning threads, by which it suspends 
itself from the mangrove-roots; and our own 
freshwater Snails have the power of suspending 
themselves in the same manner. Mr. Warington, 
in an interesting paper on the habits of some 
aquatic animals kept in confinement, thus records 
the curious fact :— 
“In watching the movements of the Limnet, 
I was for some time under the impression that they 
had a power of swimming or sus- 
taining themselves in the water, 
as they would rise from the bottom 
of the pond, a portion of the rock- 
work, or a leaf of the plants, and 
float for a considerable period, 
nearly out of their shells, with- 
out any apparent attachment, and 
by the contortions and gyrations 
of their body and shell, move 
some little distance, inahorizontal 
direction, from the point which they had left. 
On more carefully watching this phenomenon, 
however, I found they were attached by a thread 
or web, which was so transparent as to be alto- 
gether invisible, and which ae could elongate in 
a similar way to the spider: they also possessed the 
power of returning upon this thread by gathering 
it up, as it were, and thus drawing themselves 
back to the point which they had quitted. These 
facts were clearly proved in the following manner : 
A Limneus stagnalis had glided its way along a 
young and short leaf of the Vadlisneria, which ter- 
minated below the surface of the water, and having 
LIMNEUS AURICULARIS, 
