286 SOFT-BODIED ANIMALS— THE MOLLUSKS 



Class — Amphineura 



This class of mollusks seems to show a lesser degree of specialization 

 than any other in the phylum and is, therefore, of interest in establishing 

 possible relationships between this phylum and others. Members of this 

 class have a ladder-like nervous system such as was found in planaria 

 and a ciliated larva which is similar to that produced by Neanthes. Em- 

 bryonic development of the flatworms, annelids, and Amphineura shows 

 that the cells divide in the same way and form similar structures up to a 

 certain point before beginning development in different directions. These 

 facts would indicate a rather close relationship between these three phyla. 



A typical genus is Chiton which crawls around among the rocks on the 

 seashore eating the algae growing there. Its body looks like a broad 

 flattened worm, but is covered on the dorsal surface with a series of eight 

 plates which have articulating surfaces and give more freedom of move- 

 ment than is found in other mollusks with a rigid shell. 



Class — Scaphopoda 



This is a minor class of marine mollusks that have a tubular shell 

 open at both ends. Its members are commonly called toothshells be- 

 cause of the similarity of the shells to the teeth of some large animals. 



Classification and Derivation of Scientific Words 



Phylum Mollusca (L. mollis, soft; refers to the body parts). 



Class A. Pelecypoda (Gr. pelckys, hatchet; pons, foot; refers to the 



shape of the foot). The bivalves, such as clams and oysters. 

 Class B. Gastropoda (Gr. gastcr, stomach; pous, foot; the stomach is 



in the foot). Snails, slugs, conch, abalone, etc. 

 Class C. Cephalopoda (Gr. kephale, head; pons, foot; the foot is 



wrapped around the head). Squid, octopus, chambered 



nautilus. 

 Class D. Amphineura (Gr. amphi, both; neuron, nerve; there are two 



nerves running down the body). Chiton. 

 Class E. Scaphopoda (Gr. skaphe, boat ; pons, foot ; refers to the shape 



of the foot). The tooth-shells. 



REVIEW QUESTIONS 



1. Give the characteristics of the Mollusca. 



2. What is the economic importance of the fresh-water clam ? 



3. How can you estimate the age of a clam? 



4. Describe the feeding habits of a clam. 



