Chap. 31 MOLLUSKS — specialists in security 635 



Commonly chitons are about four inches long. They are generally drab- 

 colored, frequent shaded rocks and seaweed, and are neither dangerous, 

 strikingly beautiful, nor edible. They are plant feeders that scrape fine bits 

 from the rocks and seaweedsand the small ones furnish picking for shore birds. 

 Chitons are chiefly interesting as pieces of living history. 



Chitons are bilaterally symmetrical outside and inside. The shells and a 

 fold of the mantle project over deep grooves, one extending along each side of 

 the under surface of the body. These are parts of the mantle cavity with gills 

 located in them as they are in the larger mantle cavity of clams. The surface 

 of the tongue is a file, in function a replica of those on the tongues of snails. 

 The relatively large fleshy foot has a ciliated surface and strong muscles. 

 Chitons move like drifting sailboats. 



Class Scaphopoda — Tooth Shells 



This is a small and little known class of mollusks, with single shells usually 

 less than two inches, but in some species even six inches long (Fig. 31.1). 

 They live in sand beyond the low tide mark, some of them at great depths. 

 Their shells are open at both ends, larger at the head end which is pointed 

 forward as they burrow. 



Class Pelecypoda — Bivalves 



These are the clams, oysters, scallops, and other two-shelled mollusks. The 

 majority of fresh-water bivalves, both large and small, are widely distributed 

 in lakes and streams (Fig. 31.5), All bivalves are essentially similar and the 

 following outline of the fresh-water mussel applies in general to common 

 marine species such as the round clam, Venus, and the soft shell, Mya. Fresh- 

 water mussels practically all belong to one family. Over 500 species have been 

 found in the United States, but many are impossible to distinguish except by 

 special students of this group. 



* 



Fresh-water Mussel 



Skin and Mantle. The mantle makes the shell; the shell protects the mantle 

 and together they are the main contributors to security which is the prime 

 achievement of mollusks (Figs. 31.6 and 31.7). The mantle is the soft cover- 

 ing of the body extended into folds on the ventral side, opposite the hinge of 

 the shell. It covers the back of the clam and folds of it hang free in front as an 

 open topcoat hangs free on the human body. It is different in that the mantle 

 of the mussel also fits close to the body even though the folds hang free. There 

 is a space between the open coatsides and the human body. The comparable 

 space in the clam is the mantle cavity in which the gills are suspended. The 

 borders of the mantle contain many glandular cells, are supersensitive to touch, 



