CHAPTER THIRTY-FOUR 



MOLLUSCA 



1. THE Mollusca include such familiar animals as the Characters 



.. i r 1 11 -i 'i of Mollusca 



snails and slugs, cuttlefish, oysters, and clams. 1 hey 

 are soft-bodied, without the chitinous external armor of 

 the Arthropoda. The majority of the species possess a 

 shell, which is secreted by the animal, and consists prin- 

 cipally of carbonate of lime. The alimentary canal, 

 blood system (with a simple heart), respiratory system 

 (lung or gills), and liver are well developed, and eyes are 

 usually present. Some feed on vegetable matter, others 

 are carnivorous. The number of known species is very importance 

 great, and as the shells are readily preserved as fossils, 

 mollusks are of great importance to the geologist. In 

 the course of time the groups of mollusks have become 

 variously modified and consequently their fossil remains 

 serve as excellent guides to the strata, each considerable 

 layer of rocks having its own characteristic assemblage. 



2. The larger groups of mollusks are so different that Groups of 

 at first they seem to have little in common. Compare, Mollusca 

 for instance, a snail with a clam or an octopus. There 



is, however, a certain similarity of structure which leads 

 us to place them all in a single phylum, and sometimes 

 very different-looking forms are found to be connected 

 by intermediates. Thus the snail and the slug, al- 

 though very distinct, are merely the extremes of a series 

 of species in which the shell is of all sizes, grading from 

 the larger one of the typical snail to the rudimentary or 

 hidden one of the slug. Finally, in some slugs, not even 

 a rudiment of the shell remains. The large group called 

 Gastropoda (literally stomach-footed) includes the or- 

 dinary coiled shells - - terrestrial, fresh-water, and ma- 

 rine - - and the naked slugs. If we examine an ordinary 



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