MOLLUSCA. 401 



in water. In some cases the hairs may be considered as 

 united, and forming continuous crusts or ridges. These 

 hairs, as well as the cuticle, are liable to be worn off, and 

 in some places can seldom be perceived, unless in early age. 



The most important appendix to the skin appears to be 

 the shell. This part is easily preserved, exhibits fine forms 

 and beautiful colours, and has long occupied the attention of 

 the conchologist. The matter of the shell is secreted by 

 the corium, and the form which it assumes is regulated by 

 the body of the animal. It is coeval with the existence of 

 the animal, and appears previous to the exclusion from the 

 egg ; nor can it be dispensed with during the continuance 

 of existence. The solid matter of the shell consists of car- 

 bonate of lime, united \\ ith a small portion of animal mat- 

 ter, resembling coagulated albumen. 



The mouth of the shell is extended by the application of 

 fresh layers of the shelly matter to the margin, and its thick- 

 ness is increased by a coating on the inner surface. These 

 assertions are abundantly confirmed by the observations of 

 REAUMUR*, whose accurate experiments have greatly contri- 

 buted to the elucidation of conchology. If a hole be made in 

 the shell of a snail, and a piece of skin so glued to the inner 

 margin as to cover the opening, the shelly matter will not 

 ooze out from the broken margin of the fracture, and cover 

 the outside of the skin, but will form a coating on its inner 

 surface, thus proving it to have exuded from the body of 

 the animal. When a portion of the mouth of the shell of a 

 snail is broken off, and a piece of skin glued to the inner 

 margin, reflected outwardly, and fixed on the body of 

 the shell, the defective part is again supplied, and the mat- 

 ter added to the inner surface of the skin, thus leaving the 

 interposed substance between the new formed portion and 



" Memoires de V Academic des Sciences, 1709. 

 VOL. II. C.C 



