CHAPTER XX 

 PHYLUM MOLLUSCA 



Molluscs are bilaterally symmetrical, unsegmented ani- 

 mals which usually have a ventral muscular foot and 

 commonly possess a calcareous shell. All have a mantle 

 of some sort. This is a covering which may have various 

 uses, such as protecting the body, forming the shell, and 

 swimming. Between the mantle and the body proper there 

 is a space, the mantle cavity, which is of great importance in 

 the activities of molluscs. There are five molluscan classes, 

 but only three are of general importance. 



Class 1. Gastropoda; snails, slugs, nudibranchs; with a large flat 

 creeping foot, and usually having a spirally coiled shell. 



Class 2. Lamellibranchiata: clams, oysters, ship worms; with an 

 axe-shaped foot and a shell composed of two flat valves. 



Class 3. Cephalopoda: squids, cuttle-fishes, devil-fishes, octopi; 

 with the body elongated and the foot divided into a number of arms, 

 which usually bear suckers; shell usually small and inclosed within the 

 mantle, or absent. 



CLASS 1. GASTROPODA 

 THE POND SNAIL, Physa gyrina Say 



Self -maintenance. Physa (Fig. 86) lives among aquatic 

 plants where there is an abundance of food. It moves 

 about by making muscular movements of its broad ventral 

 foot. If a Physa is observed from beneath while crawling 

 in a glass dish, little waves will be seen to follow each other 

 from one end of the body to the other. These indicate 

 bands across the body where the foot is temporarily pulled 

 away from the glass and moved forward a little. This is a 

 slow method of progress, but sure. As the snail moves 

 along a slimy trail is left behind, and the slime is of consid- 



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