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 branchiae, 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 excretory 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 

 branchiae, 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- 



