MOLLUSKS. i Si 



however, the animals receive an equal share of attention from the 

 naturalist ; and the arrangement of their hard persistent coverings 

 depends almost entirely upon the structure of the animals which 

 formed them. 



Mollusks may be defined as soft and fleshy animals, devoid of 

 bones or any internal skeleton, and not divided, like insects and 

 worms, into rings or articulations. Their body is covered with 

 an irritable and contractile skin, which is moistened by a viscid 

 liquor that exudes from it, and which is in very many instances 

 ample enough to form folds that envelope the creature more or 

 less completely as in a mantle or cloak. In some cases this skin 

 is naked, and then the mantle is thick and viscous ; in the greater 

 number, however, it is protected by a hard covering, called a shell, 

 beneath which the mantle is thin and transparent. Their most 

 essential character, however, lies in their nervous system, which 

 consists of a certain number of nervous centres or ganglia, from 

 which the nerves are given off to different parts of the body. 

 These ganglia are principally concentrated around the entrance 

 to the alimentary canal, and form a collar or ring, that surrounds 

 the oesophagus or throat, and is connected with other ganglia, 

 disposed without symmetry among the viscera, or in the neighbour- 

 hood of the organs of locomotion. From this unsymmetrical con- 

 dition of the nervous centres, the whole class has received the 

 name of Heterogangliata.* Many of the Mollusks are terrestrial, 

 and breathe the air ; but the greater part live entirely in the 

 water, from which they derive their nutriment, and in which they 

 breathe by the aid of branchiae, or gill-like appendages. Those 

 which are terrestrial are seen in our gardens, pastures, and plan- 

 tations, on the trunks and stems of trees, and in moist and shady 

 places ; while multitudes of the aquatic species are to be found 

 in the seas both of tropical and arctic regions, as well as in those 

 which environ our own islands. Others dwell in lakes and ponds 

 of fresh water, or live at the bottom of rapid streams ; some arc 

 amphibious. In short, they may be said to be universally distri- 

 buted 'wherever circumstances permit of their existence. 



The number of species already in museums probably reaches 

 8,000 or 10,000. There are cabinets of marine shells, bivalve and 

 univalve, which contain from 5,000 or 6,OOO, and collections of 

 land and fluviatile shells which count as many as 2,000. The 

 total number of Mollusks, therefore, probably exceeds 15,000 

 species. The Mollusks can only be studied properly in a living 



* ere/oos, heteros, dissimilar ; ydvy\ioi>, ganglion, a ganglion. 



