itch'' snails have been almost completely eliminated even from some of the 
larger lakes. Other molluscicides are sodium pentachlorophenate, copper 
pentachlorophenate, and dinitro-ortho-cyclohexylphenol. 
GENERAL INTRODUCTION TO THE MOLLUSCA 
The Phylum Mollusca, which includes the snails, is a large group of ani- 
mals of extremely divergent form, second only to the arthropods in number 
of described species. Most mollusks have a hard external shell (although 
there are well-known exceptions such as the slugs and squid), a muscular 
saclike covering (the mantle) which secretes the shell, soft bodies with no 
segmentation (except for two deep-sea species) or internal skeleton, numer- 
ous mucous or slime glands, and a large ''foot'' that is variously modified 
for crawling, digging, or grasping prey. The shell, often of prime impor- 
tance in species determination, is mainly composed of calcium carbonate 
(lime) and functions to protect the animal from injury, predators, desic- 
cation, and other unfavorable conditions of the environment. The great di- 
versity in structure, ornamentation, and color of mollusk shells has long 
made them a favorite of collectors. 
Because of their great diversity and large numbers (nearly 100, 000 species), 
mollusks are found in nearly all regions and habitats on earth. They are 
found in deep-sea dredgings, in sandy, shallow lagoons, on coral reefs, in 
arctic waters, hot springs, in both tidal and freshwater mudflats, in swift 
mountain streams, deep lakes, temporary wood pools, inthe ground, on 
tops of high snow-covered mountains, in crater lakes, deserts, trees, 
densely populated urban areas, and even on and in other animals as para- 
sites. They eat every possible food, from soil micro-organisms, plankton, 
poisonous mushrooms, cactus plants, garden crops, refuse, paper, and 
fish, to living or decayed land animals and their excrement. Some are even 
cannibals, eating their own species. With such diversification it is small 
wonder that many are of considerable economic importance to man. 
The importance of the foot in recognizing the various assemblages of re- 
lated species within the phylum is seen in the names given the principal 
groups. There are six such groups, or classes, in the Mollusca (fig. 1). 
Each class, except the chitons (Amphineura) and the two species of seg- 
mented mollusks (Monoplacophora), has a name which bears reference to 
the foot, that is, Cephalopoda, Gastropoda, Scaphopoda, and Pelecypoda. 
Cephalo refers to head; gastro to stomach; scapho to digging; pelecy to axe; 
and poda to foot. 
In addition to modifications of the locomotor organ, these groups are fur- 
ther distinguished by other features such as basic structure of the shell, the 
absence, presence, or degree of development of the head, the degree of 
development of the nervous system and sense organs, modifications of the 
respiratory organs, and by structure of the radula or teeth. 
