MOLLUSKS 115 
many species lay eggs. Many snails are herbiverous, while a few 
prey upon barnacles or other mollusks. Sense organs are poorly 
developed in snails, but in land snails and slugs we often find eyes 
mounted upon the tip ends of long tentacles which arise from the 
head. In sea snails, however, the eyes are smaller, and are found 
at the bases of the tentacles or only half way up. 
The clams, oysters and mussels form a great group called the 
“Tamellibranchiata,’ for their gills project in curtain-like sheets 
from the sides of the body, within the shell. In these mollusks we 
find two shells or more properly ‘“‘valves,” one on the right and the 
other on the left side of the body, while the “hinge” between the 
valves extends along the back of the animal. This hinge is tough 
and muscular, and tends constantly to open the valves, but this is 
prevented by the contraction of two powerful sets of muscles which 
run across from one valve to the other. 
The foot of the animal is often well developed and capable of 
pushing the creature rapidly through the sand, as in the razor- 
clam, while in other cases, as in the oyster, it is small and degenerate. 
There is no distinct head, but the mouth has two leaf-like lips. It 
should be said that the long, muscular “neck” of the soft-shell clam 
is not the neck of the animal, but is a tubular outgrowth of the 
mantle, which opens by two apertures. ‘The one on the lower side 
is for the admission of water to the gill chambers, and the other is 
the analaperture. A continuous current of water flows in at the for- 
mer and out from the latter aperture. In scientific language the 
“neck” of the clam is called the siphon, and it is well to remember 
that it extends outward from the posterior end of the body; the 
mouth being at the opposite side of the shell. 
Clams, oysters and mussels feed upon minute plant and ani- 
mal organisms, which are drawn in between the edges of the mantle, 
or through the siphon by means of the constant beating of vast 
numbers of little hair-like cilia that cover the gills. 
The most highly developed mollusks are the Cephalopoda, 
represented by the squid, octopus, nautilus, and argonaut. In 
these we find two large eyes and eight or ten long, muscular, 
sucker-bearing ‘‘arms” which surround the mouth. These arms 
constitute a portion of what was once the foot of the ancestral forms 
from which the Cephalopoda are descended. ‘The remaining part 
