THE CEPHALOUS MOLLUSKS. Za 
our corkscrews, and all our screws, the thread wound on our reels, 
and fifty other examples, all show the;tendency we naturally have 
to motion from right to left. a 
Just as occasionally we have a left-handed screw, so there are 
a few—but very few—shells whose spirals turn from left to right; 
but these are only exceptions. Sometimes the right spiral shells 
become left by a freak of nature; when this is the case, they are 
highly valued by collectors ; and, on the other hand, the left-handed 
Testacea sometimes become right-handed—thus showing a disposi- 
tion to return to the normal condition of the race. Some shells 
are not twisted into spirals—for example, the Paéel/lz, which 
resemble large extinguishers—but, when they are young, there is 
a very evident twist exhibited, so that even these are no exception 
to the rule. 
There are some univalves which float gracefully on the surface 
of the water, unaffected by the billows. The beautiful little shell- 
JANTHINA COMMUNIS. 
fish, the Y¥anthina, is one of these; it is covered by a thin, fragile 
tunic, of a delicate violet tint. It is suspended to a spongy mass, 
composed of cartilaginous vesicles, not unlike a lump of soap-suds 
solidified ; this is the float which buoys it up. At the slightest 
alarm the janthina ejects a quantity of dark red liquid, and sinks 
to the bottom. Some naturalists think that this liquid distends its 
swimming apparatus, and that upon ejecting this it sinks. 
There are mollusks which spread out a thin veil attached to 
their mantle, which the wind catches, and so they sail over the 
surface of the seas. But the greater part of the univalves live 
always submerged—some of them at great depths. They have 
been brought up from a depth of 1,400 fathoms, 
The mollusks which live in shells, are either found on the marine 
