TESTACEOUS FISHES. 



'077 



radical error, we have volumes written upon 

 the subject of shells, and very little said on 

 the history of shell-fish. The life of these 

 industrious creatures, that for the most part 

 creep along the bottom, or immoveably wait 

 till driven as the waves happen to direct, is 

 almost entirely unknown. The wreathing of 

 their shells, or the- spots with which they are 

 tinctured, have been described with a most 

 disgusting prolixity ; but their appetites and 

 their combats, their escapes and humble arts 

 of subsistence, have been utterly neglected. 



As I have only undertaken to write the his- 

 tory of Animated Nature, the variety of shells, 

 and their peculiar spots or blemishes, do not 

 come within my design. However, the man- 

 ner in which shells are formed is a part of 

 natural history connected with my plan, as 

 it pre-supposes vital force or industry in the 

 animal that forms them. 



The shell may be considered as a habita- 

 tion supplied by nature. Ft is a hard stonv 

 substance, made up somewhat in the manner 

 of a wall. Part of the stony substance the 

 animal derives from outward objects, and the 

 dnids of the animal itself furnish the cement. 

 These united make that firm covering which 

 shell-fish generally reside in till they die. 



But, in order to give a more exact idea of 

 the manner in which sea-shells are formed, 

 we must have recourse to an animal that lives 

 upon land, with the formation of whose shell 

 we are best acquainted. This is the garden- 

 snail, that carries its box upon its back, whose 

 history Swammerdam has taken such endless 

 p^ins to describe. As the manner of the for- 

 mation of this animal's shell extends to that 

 of all others that have shells, whether they 

 live upon land or in the water, it will be 

 proper to give it a place before we enter 

 "upon the history of testaceous fishes. 



To begin with the animal in its earliest 

 state, and trace the progress of its shell from 

 the time it first appears. The instant the 

 young snail leaves the egg, it carries its shell 

 or ks box on its back. It does not leave the 

 egg till it is arrived at a certain growth, 

 when its little habitation is sufficiently har- 

 dened. This beginning of the shell is not 

 much bigger than a pin's head, but grows in 

 a very rapid manner, having at first but two 

 circumvolutions, for the rest are added as the 



snail grows larger. In proportion as the ani- 

 mal increases in size, the circumvolutions of 

 the shell increase also, until the number of 

 those volutes come to be five, which is never 

 exceeded. 



The part where the animal enlarges its 

 shells is at the mouth, to which it adds in 

 proportion as it finds itself stinted in ittf 

 habitation below. Being about to enlarge 

 its shell, it is seen with its little teeth biting 

 and clearing away the scaly skin that grows 

 at the edges. It is sometimes seen to eat 

 those bits it thus takes off; at other times it 

 only cleans away the margin when covered 

 with films, and then adds another rim to its 

 shell. 



For the purposes of making the shell, 

 which is natural to the animal, and without 

 which it could not live three days, its whole 

 body is furnished with glands, from the ori- 

 fices of which flows out a kind of slimy fluid, 

 like small spiders'threads,which join together 

 in one common crust or surface, and in time 

 condense and acquire a stony hardness. It 

 is this slimy humour that grows into a mem- 

 brane, and afterwards a stony skin; nor can 

 it have escaped any who have observed the 

 track of a snail, that glistening substance 

 which it leaves on the floor or the wall, is no 

 other than the materials with which the ani- 

 mal adds to its shell, or repairs it when bro- 

 ken. ;.) 

 Now to exhibit in a more satisfactory man- 

 ner the method in which the shell is formed. 

 The snail bursts from its egg with its shell 

 upon its back ; this sRell, though very simple, 

 is the centre round which every succeeding 

 convolution of the shell is formed, by new 

 circles added to the first. As the body of the 

 snail can be extended no where but to the 

 aperture, the month of the shell only can, of 

 consequence, receive augmentation. The 

 substance of which the shell is composed is 

 chiefly supplied by the animal itself, and is 

 no more than a slimy fluid which hardens 

 into bone. This fluid passes through an in- 

 finite number of little glands till it arrives at 

 the pores of the skin ; but there it is stopped 

 by the shell that covers the part below ; and 

 therefore is sent to the mouth of (he shell, 

 where it is wanted for its enlargement. There 

 the first layer of slime soon hardens; awl 

 5C 



