ORDER III. TESTACEA, 319 



STROMBUS, MUREX, TROCHUS, TURBO, HELIX, NERITA, 

 HAHOTIS, PATELLA, DENTALIUM, SERPULA, TEREDO, 



and SABELLA. 



In order to assist the young reader to acquire some in- 

 sight into the formation of shells, a few observations shall 

 be here added on the garden-snail ; with whose figure 

 every one is acquainted, and whose history has been very 

 minutely investigated. As nature acts in an uniform man- 

 ner, with regard to this animal and all other testacea, 

 whether they belong to the sea, to the land, or to rivers, 

 the history of one will, of course, be the history of all 

 the rest. 



At the instant when the young snail quits the egg, its 

 shell is seen on its back : at first this is not much larger 

 than the head of a pin, with two circumvolutions ; but 

 it enlarges very rapidly. In proportion as the animal 

 grows, the spiral turns increase, till they reach the number 

 of five, at which they remain fixed. 



From the mouth the snail augments its shell, accordingly 

 as it finds itself strained for room beneath ; and, when about 

 to extend its covering, it may be seen biting and clearing 

 away the scaly skin that adheres to the edges with its 

 little teeth, after which it adds another rim to its abode. 



The art of manufacturing shells is natural to the snail, ' 

 as well as to other testaceous animals. For this purpose 

 its whole body is furnished with glands, from the orifices of 

 which exudes a kind of slimy fluid, which unites in one 

 common crust or surface, and, in time, acquires a stony 

 hardness. That glistening substance, in fact, which the 

 snail so frequently leaves behind it, is, in reality, the 

 matter with which the animal either augments its shell, or 

 repairs its defects. Thus every shell may be considered 

 as a composition of layers of slime, originally proceeding 

 from the creature's body, and moulded conformably to its 

 shape and various exigencies. 



But, though the actual formation of shells may be thus 

 accounted for, they, probably, receive their beautiful tints 

 F 4 



