MOLLUSCA. 239 



^vhicll tliey frequently reside, and hence are generally 

 known by the name of '• Shell-fish." Formerly the 

 animals which formed and inhabited these shells were 

 little known, and, consequently, little attended to. 

 The shells alone attracted the attention of the 

 student, and hence the study of this branch of natural 

 science received the name of Conchology,* and the 

 appellation is still in general use. At the present 

 day, 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 tiie animals ^vhich 

 formed them. 



jMollusks 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 articu- 

 lations. 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 sivin 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 ivithout symmetry among 

 the viscera, or in the neighhourhood of the organs of 

 locomotion. From this unsymmetri(,*al condition of 

 the nervous centres, the ^vhole class has received the 

 name of Heterogangliata. t Many of the J\Iollusca 



* Kouxn, conclie, a shell : Xoyos, logos, a d/'sccmrse. 



+ erepos, bcteros, dissiiiiilar -^avyKLou, ganglion, a ganglion. 



