XXI. THE MOLLUSKS 



Problem XXXIII ( Optional). A study of mollusks and their 

 enemies with reference to their economic importance. (Labora- 

 tory Manual, Prob. 



To the average high school pupil a clam or oyster on the " half shell " 

 is a familiar object. The soft " body " of the animal lying between the 

 two protecting " valves " of the shell gives the name to this group (Latin mol- 

 lis sof t) . Most mollusks have a limy shell, either bivalve (two-valved) , as 

 the oyster, clam, mussel, and scallop, or univalve, as in the snail. Usually 

 the univalve shell is spiral in form, some of nature's most beautiful objects 

 being the spiral shells of some marine forms. Still other mollusks, for 

 example, the garden slug, have no external shell whatever. 



This limy shell envelope when present, is formed from the outer edge and 

 surface of a delicate body covering called the mantle. The mantle may be 

 found in the opened oyster or clam 

 sticking close to the inside of the 

 valve of the shell in which the body 

 rests. Between the mantle and the 

 body of the clam or oyster is a space, 

 the mantle cavity. In the space hang 

 the gills, platelike striated structures. 

 By means of cilia on the inner surface 

 of the mantle and on the gills a con- 

 stant current of water is maintained 

 through the mantle cavity bearing 

 oxygen to the gills and carbon dioxide 

 away. This current of water passes, 

 in most mollusks, into and out from 



the mantle cavity through the siphons, the muscular tubes forming the 

 " neck " of the " soft clam " being an example of such an organ. 



The food of clams or oysters consists of tiny organisms, plant and 

 animal, which are carried in the current of water to the mouth of the 

 animal, this water current being maintained in part by the action of cilia 

 on the palps or liplike flaps (p. 269) surrounding the mouth. A single 

 muscular foot aids in locomotion when the animal moves about. Many 

 mollusks, as the oyster, are fixed when adult. 



The shallow water of bays and other quiet bodies of salt water where 

 clams and oysters live, literally swarm with tiny plants. The conditions 

 for the growth of such plants is ideal. Water from the rivers contain- 

 ing organic waste and depositing daily its load of mud on the bottom 



267 



Fulgar, a univalve mollusk common in 

 Long Island Sound, which does much 

 harm by boring into the shells of 

 edible mollusks. 



