320 MARINE INVERTEBRATES 
the mantle recurves. When the creature is disturbed or is at 
complete rest it may withdraw that portion of the mantle which 
protrudes from below the shell entirely within the edge of the 
shell, so that the “soft parts” may be wholly covered, and thus 
protected from harm. In such a state of rest nothing may be 
seen from above but a shell. The entire upper surface of the 
mantle is firmly attached to the under or inner side of the shell, 
except the mantle margin, this being free. 
The nervous system consists of a circumesophageal ring of 
nerve-cells, with two cords passing along the digestive tract to 
the posterior end of the animal. In the head and above the 
buccal mass are several series of nerve-ganglia. These also com- 
municate with the posterior portion of the animal through two 
parallel cords which pass along its ventral side. From these 
four principal branches, which traverse the entire length of the 
creature, minor nerve-cords are thrown off to the tentacles, the 
foot, the branchie, and the various organs of the viscera. The 
sexes are separate. 
Reviewing our “typical mollusk,” we find that it is bilaterally 
symmetrical—that is, uniform upon both sides of a median line 
from mouth to eer opening; is unsegmented; has the ven- 
tral surface thickened into a creeping foot or disk; and is pos- 
sessed of a head with tentacular processes, a radula, lateral 
branchiz, a heart with a ventricle and two auricles, a well-devel- 
oped mantle, and a shell. 
If the reader can learn these few details of organization he 
knows the structure of a mollusk. In turning to the actual 
living forms he will find merely modifications (though sometimes 
confusing ones) of these various parts. 
CLASSIFICATION 
The Mollusca are divided into five great classes: Amphineura, 
Gasteropoda, Scaphopoda, Pelecypoda, and Cephalopoda. These 
names, it will be noticed, with the exception of the first, all refer 
to the foot, the modifications of the foot being selected as the basis 
of the first great divisions of the phylum. The Gasteropoda crawl 
upon the thickened ventral surface of their bodies (already re- 
