166 ANIMAL ACTIVITIES. 
Another important class of mollusks is the Cephalop- 
oda. The squid belongs to this class, as do the 
Fic. 130.—A Squid. P, 
its pen. 
octopus, the argonaut, and the 
pearly nautilus. 
Characteristics of Mollusca. 
The name Mollusca is given 
on account of the fact that the 
bodies of these animals, ex- 
cepting the shell, have no hard 
parts. In Mollusca the body 
is typically bilateral. There is 
a mantle present which in most 
cases secretes a shell which is 
as much a part of the animal 
as is the mantle itself. These 
shells increase in size as the 
animal grows, and .so exhibit 
lines of growth. A fleshy foot 
is also present. The nervous 
system consists of three pairs of ganglia connected by 
nerves. Eyes are present in many forms and in the 
higher cephalopods they closely re- 
semble the eyes of Vertebrates. In 
all but the Pelecypoda the mouth is 
provided with a lingual ribbon, which 
is a sort of rasp in the mouth by 
which in many cases thick shells may 
be bored entirely through. 
Questions. How do the sense- 
organs of mollusks. which forage for 
food compare with those of sedentary 
mollusks ? 
What protective devices do you 
observe among mollusks ? 
How can one compel an oyster to 
produce a pearl ? 
Topics for Reports. Tyrian Dyes. 
BIQIRE 
IVI ne 
A40 
Sta 
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r 
aX 
Fic. 131.—Part of 
a Lingual Rib- 
bon (magnified). 
Eye-stones. 
The Octopus and Squid. The Chambered Nautilus. 
