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Mollusles — Specialists in Security 



Most people know that clams and oysters make shells; that oysters belong 

 in stew, clams in chowder, and that scallops are fried. Many know the pleasant 

 softness of oysters on the half shell. When the novelist Thackeray ate his first 

 raw oyster he is said to have exclaimed that he felt as if he had "swallowed a 

 little baby." For the majority of mollusks, these impressions are correct. Most 

 of them bear shells, provide abundant food for man and other animals, and 

 have such soft bodies that the phylum is named Mollusca. 



The group includes an enormous number of animals whose lives are deeply 

 affected by their shells. It contains animals of such different forms and activity 

 as snails and slugs, clams and oysters, swift darting squids, slow creeping 

 chitons, and the storied paper sailor and chambered nautilus (Fig. 31.1). 

 Mollusks are scattered over the lands and through the seas and fresh waters 

 of the world. There are over 80,000 known species. Fossils of the ancestral 

 mollusks are abundant in Lower Cambrian rock laid down 600 million years 

 ago. The free-swimming ciliated larvae of mollusks and annelid worms are so 

 similar that they suggest a common ancestry. 



General Characteristics. The dominant structures of mollusks are the mantle, 

 the foot, and the spiral form. The fleshy cloaklike mantle produces the myriad 

 kinds of shells, takes part in forming the gills and the lung sacs of air-breathing 

 snails, and in many species bears cilia. The foot is the organ of locomotion 

 (Fig. 31.5), the traveling platform of snails, the digging tool of clams, the 

 head-foot from which tentacles originate in squids. Spirals or some hint of 

 spirality appear in many mollusks; laterally developed spirals are prominent 

 in the majority of snails; the symmetrical spiral is equally prominent in the 

 chambered nautilus; and an oblique slant in the hinges of clam and oyster 

 shells is noticeable. Mollusks have a true coelom but lack several prominent 

 features of other higher invertebrates. Although they can swim, crawl, climb, 

 dig and bore, they have no legs. The body is not divided into segments, and 

 only in the chitons is the shell segmented (Fig. 31.1). 



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