236 THE WORLD OF THE SEA. 
motion. In some species, the cavity is scarcely s$oth of an inch in 
diameter, and yet it contains fifty or sixty of these stones. Their 
sense of smell resides in the surface of the tentacles. The 
mollusk is provided with two of these appendages. In our 
organism the nasal cavities open into the respiratory canal, so 
that the air which the expansion of the lungs causes to rush 
through the nose, carries to the olfactory nerve the particles of 
bodies which produce the sensation of smell. In those univalves 
where the nasal organs are separate and distinct from those of 
respiration, they are endowed with great mobility, and so, as it 
were, go in search of the particles which are to cause them the 
sensation. 
The tentacles are covered with a microscopic down—very short 
Am 
ey 
NERITA POLITA. 
hairs, which are in restless movement, like vibratory cils. Their 
office seems to be to produce currents which bring the particles 
of matter which the water contains in contact with the surface of 
the organs. 
The marine cephala deposit their eggs singly or in masses. 
When in this latter manner, they cluster them in vast numbers 
in every possible shape. As varied as the shapes are their colours, 
which pass through every tint save those which are allied to blue. 
Many of them are encased in a gelatinous substance, more or 
less transparent, and enveloped in a membrane. Some of the 
univalves carry their eggs deposited on their own shell. The 
eggs of the Yanthina are suspended in the interstices of that 
wonderful swimming-bladder which we have described.  Fre- 
quently more than a million are here deposited. 
Some naturalists, especially of the French school, have been 
much interested in speculating as to the tender feelings which 
