THE CEPHALOUS MOLLUSKS. 235 
of these mollusks pierce the shell of a pecten, and devour the 
animal within it. 
The nourishment of the bivalves is determined by their organi- 
sation; some are herbivorous, others are carnivorous. The very 
small species live on microscopic vegetables or animalcules. The 
common periwinkles, of which we shall presently speak, live upon 
those infinitesimal plants which give off myriads of impalpable 
spores, which are held in suspension in the water. If these minute 
plants and their spores be placed in an aquarium, in a very short 
time the walls of the vessel will become coated with a green film, 
a boundless forest of these vegetables. But if we also place in 
the aquarium one or two little gasteropods, they soon arrest the 
vegetable growth, and restore the crystal transparency of the 
water. 
The cephalous mollusks possess a brain peculiarly charac- 
teristic. The noble organ is not placed in the head, but in the 
neck. It is a ring of nerval matter, which surrounds the beginning 
of the digestive tube. Generally, this ring is loose and movable ; 
it advances or recedes according to the movements of the animal. 
Sometimes it is very forward, sometimes it is further back; now 
it enters the head, and again it retreats into the body ; its position 
is fixed by the will of the mollusk. It is usually found at the 
nape of the neck. This may explain the fact how a snail will live 
after its head is cut off—the brain, the great centre of the motive 
power of life, not being injured by that formidable operation. The 
brain varies its colour in different species; it is found white, 
yellow or orange, and black. 
The univalves enjoy the senses of sight, of hearing, and of 
smell. The eyes are placed upon prominences at the base of the 
tentacles. They are simple in their construction, not made for 
long vision, nor yet for the blaze of the full light of day. The 
auditory organs are situated at the bottom of the neck. They are 
not visible from without ; they have neither that external append- 
age which we term the ear, nor yet is there a visible orifice. 
These curious organs are membranous and transparent pouches, 
filled with a very limpid liquid, which holds in suspension some 
minute stones; these are endowed with a peculiar trembling 
