82 



Shell-fish form two great families ; that of the con- 

 chce, or larger kind, whose shell is made up of two, or 

 several pieces; and that of snails, whose shell con- 

 sists of one single piece, turned, for the most part, spi- 

 rally. 



The structure of the first seems much more simple 

 than that of the last. The conchx have neither head, 

 horns, nor jaws ; one can uuly obcorvp in them air-vents 

 a mouth, an anus, and sometimes, a sort of foot. Tiie 

 greatest part of snails ; on the contrary, have a head, 

 horns, eyes, a mouth, an anus, and a foot. The round 

 anci fleshy head is at the anterior, and upper part of the 

 animal. It contains a brain, composed of two littlt 

 globes, whose apparatus is of such a rnoveable nature, 

 that it is transferred from the hinder to the fore part, at 

 the pleasure of the snail. -The horns, which are two, or 

 four, in number, placed on the sides of the head, are a 

 kind of pipes, susceptible of various motions, and which 

 the animal can draw into his head by the help of a 

 muscle ; which the Grand Observer has ordained to 

 perform the functions of the optic nerve. In some spe- 

 cies of snails, the eyes are placed at the extremity of 

 the horns, as at the end of the shank of a pair of spec- 

 tacles. In others, at the base, or towards the middle. 

 They are black and brilliant ; pretty much resembling 

 the term of a very small onion. We can only discover 

 their tunic, which is called the uvea ; but they have the 

 three humours belonging to our eye. The mouth, which 

 is commonly a small chink, like a furrow, is furnished, 

 in many species oi them, with tw cartilaginous jaws, 

 placed on each other, whose inequalities, or clefts, per- 

 form the office of teeth: some species have real teeth, 

 like those of a sea-clog, which are extremely small. 



The shell-fish that have no jaws, have a fleshy, or 

 muscular pipe, which supplies the place of a snout. 



Snails are not provided with feet ; but they have one 

 foot, of a particular make, which is nothing more than 

 a collection of a great number of muscles, whose mo- 



