MOLLUSCA. 5 



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, for such it 

 must be considered, 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 ani- 

 mal. 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 carbonate of lime, 

 united with a small portion of animal matter, 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 

 statements are abundantly confirmed by the observations of 

 Reaumur, (Memoir es de V Academic des Sciences, 1709)> 

 whose accurate experiments have greatly contributed 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 out- 

 side of the skin, but will form a coating on its inner sur- 

 face, 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 matte? 



