Phylum MoUiisca and Minor Eitcoelomate Phyla 471 



end by special muscles and is pulled back and forth over a protruding 

 piece of cartilage. 



The gastropods are an exceptionally varied and abundant group 

 of animals. The terrestrial forms vary in size from very small to 

 the large tropical arboreal forms. Some are able to withstand dry con- 

 ditions by their ability to secrete a mucous plug over the shell en- 

 trance. The fresh-water and marine snails are equally varied and abun- 

 dant. The limpets, with their caplike shells, live among the rocks of the 

 sea coast. The terrestrial slugs have the shell reduced to a mere in- 

 ternal rudiment ; the nudibranchs, sometimes called sea slugs, like- 

 wise have a much-reduced shell. 



Economically the gastropods are an extremely important group. 

 This is in part due to the destruction of vegetation by many of the ter- 

 restrial forms. They are very fond of succulent vegetables. The oyster 

 drill feeds on oyster beds, and at times inflicts great damage. Many 

 snails, as pointed out in previous chapters, are important as intermedi- 

 ate hosts for various trematode parasites. 



On the other hand, many of the snails are important as food for 

 fish, shore birds, and whales. Some of the marine forms such as 

 abalone of our west coast are popular as food, while the European 

 Helix is highly prized as a food delicacy. 



THE CLASS PELECYPODA 



The members of this class are somewhat more uniform in appear- 

 ance and anatomy than are those of the other classes of the Mollusca. 

 Commonly they are known as the bivalves, from the hinged double shell 

 which is secreted by the folded mantle. They lack cephalization and 

 most of them move slowly through the bottom material by means of 

 the hatchet-shaped foot. Only a few such as the common scallop 

 swim rapidly by clapping the two valves together. In general, 

 the bivalves have but few sense organs which are poorly developed al- 

 though the scallop again proves the exception by having a row of small 

 eyes and tactile organs along the edge of the mantle. The scallop is 

 probably the most active member of this class ; more common is the 

 slow-moving fresh-water clam or the completely sessile oyster. 



In size, the bivalves vary from tiny species of 2 to 3 mm. in diam- 

 eter to the giant Tridacna of the tropical Pacific which may attain 6 

 feet in length and weigh 500 pounds. 



