THEIR PRINCIPAL CLASSES. 7 



feet are unjointed, and furnished along their inner aspects 

 with a series of cups, which the animal can affix to foreign 

 bodies, on the principle of a cujDping-glass. 



The Pteropods are free Mollusca, which have no feet to 

 arrest their prey, nor foot to creep on ; but they float con- 

 stantly in the water, sustained and moved by means of a pair 

 of fins, which are placed at the sides of the mouth in some of 

 them, and of the neck in others. With their habit of floating 

 near the surface, you will naturally conjecture that a heavy 

 shell is incompatible, and you are right : the shell is always 

 light, thin, and transparent ; and even its form is so anoma- 

 lous, that a Conchologist soon learns to distinguish them, 

 although he may never have seen the living inhabitant. 



But the great majority of the cephalous Mollusca creep on 

 the belly, by means of a flat muscular disk, and hence they 

 are named Gasteropods. The slug, the snail, the whelk, 

 periwinkle, and limpet, are familiar examples of this class. On 

 tropical shores its representatives are found in the cowries, 

 cones, and volutes, the pride of all collectors, and in the rosy 

 cheeked conchs or spinous rock-shells brought home from 

 foreign climes by the sailor-boy, to become the favourite 

 ornament of the chimney-piece of our English parlours. 



The acephalous Mollusca form two classes : one the bival- 

 vular or Conchifera, known by its shell, which is always 

 formed of two pieces or valves, shutting against each other 

 and enclosing the animal, as the oyster and cockle ; the other 

 class affords no familiar illustrations, for it is composed of 

 creatures which are neglected and unnamed by the vulgar. 

 They have been called Tunicated Mollusca, for they are in 

 no instance covered with a shell, but merely with a leathery 

 or soft carnous tunic, in which there are two circular aper- 

 tures, one for the admission of water and food into the inte- 

 rior, and one by which the effete and excrementitious parts 

 are expelled. Cuvier enumerates two other classes of this 

 section : the Brachiopods, which are properly a subclass of 

 conchiferous Mollusca, furnished with a pair of fleshy cili- 

 ated arms, capable of being protruded beyond the circum- 

 ference of the shell, and made subservient apparently to 

 respiration as well as the attraction of food : the other class 

 is the Cirrhopods, in which the famous Barnacles and Acorn- 

 shells are included ; but as recent discoveries have proved 

 that these belong properly to annulose or crustaceous animals, 

 we must exclude them from our Molluscan assemblage. 



Such then are the names and distinctions of the primary 

 tribes of the class of animals whose history I have undertaken 

 to give you, and which it seemed necessary you should know 



