348 



HISTORY OF FISHES. 



shells, and their peculiar spots or blemishes, 

 do not come within my design. However, 

 the manner in which shells are formed is a 

 part of natural history connected with my 

 plan, as it presupposes vital force or industry 

 in the animal that forms them. 



The shell may be considered as a habita- 

 tion supplied by nature. It is a hard stony 

 substance, made up somewhat in the manner 

 of a wall. Part of the stony substance the 

 animal derives from outward objects, and the 

 Suids 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 

 pains 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 its box on its back. It does not leave the 

 egg till it is arrived at a certain growth, when 

 its little habitation is sufficiently hardened. 

 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 circumvolu- 

 tions, for the rest are added as the snail grows 

 larger. In proportion as the animal increases 

 in size, the circumvolutions of the shell in- 

 crease also, until the number of -these volutes 

 come to be five, which is never exceeded. 



The part where the animal enlarges its 

 shell is at the mouth, to which it adds in pro- 

 portion as it finds itself stinted in its habita- 

 tion below. Being about to enlarge its shell, 

 it is seen with its little teeth biting and clear- 

 ing 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 orifices 

 of which flows out a kind of slimy fluid, like 

 small spider threads, which join together in 

 one common crust or surface, and in time con- 

 dense and acquire a stony hardness. It is this 

 slimy humour that grows into a membrane, 

 <uid 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 animal adds 

 to its shell, or repairs it when broken. 



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 ; the shell, 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 mouth of the shell only can, of 

 consequence, receive augmentation. The sub- 

 stance 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 infinite 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 the shell, where it is 

 wanted for enlargement. There the first 

 layer of slime soon hardens ; and then another 

 is added, which hardens also, till in time the 

 shell becomes as thick as is requisite for the 

 animal's preservation. Thus every shell may 

 be considered as composed of a number of 

 layers of slime, which have entirely proceeded 

 from the animal's own body. 



But though this be the general opinion 

 with regard to the formation of shells, I can- 

 not avoid thinking there are still other sub- 

 stances besides the animal's own slime which 

 go to the composition of its shell, or at least 

 to its external coat, which is ever different 

 from the internal. The substances I mean 

 are the accidental concretions of earthy or 

 saline parts, which adhere to the slimy matter 

 upon its first emission. 1 By adopting this 

 theory, we can more satisfactorily account for 

 the various colours of the shell, which cannot 

 be supposed to take its tincture from the 

 animal's body, as is the usual opinion ; for all 

 the internal parts of the shell are but of one 

 white colour ; it is only the outermost layer 

 of the shell that is so beautifully varied, so 

 richly tinctured with that variety of colours 

 we behold in the cabinets of the curious. If 

 the external coat be scaled off, as Mr Argen- 

 ville asserts, all the inner substances will be 

 found but of one simple colouring ; and con. 

 sequently the animal's own juices can give 

 only one colour ; whereas we see some shells 

 stained with a hundred. 



1 Our author is mistaken in supposing that any part 

 of the shell is composed of extraneous matter. If such 

 were the case, the same species of shell would often be 

 found to differ in its composition and external appearance, 

 whereas some species are so uniformly alike, that it is 

 difficult to distinguish between two shells. 



