III.] INTRODUCTION. xlv 



of his admirable observations on the land and freshwater 

 Mollusca of France : — 



^' Les Mollusques ont des ruses et des industries, des 

 sympathies et des inimities, des guerres acharnees et 

 des amours bizarres. Beaucoup sont a la fois male et 

 femelle, et par suite pere et mere .... Malgre leur 

 apathie apparente, les Mollusques sont des etres qui ne 

 manquent pas d^intelligence. Leur vie privee et leur 

 vie commune nous montrent des details extremement 

 curieux.^^ 



CHAPTER III. 



SHELLS. 



FORMATION. COMPOSITION. SHAPE. NIJCLErS. GROWTH. 



COLOFR. DECOLLATION. EROSION. OPERCULUM. EPIDERMIS. 



Formation. — The shells of Mollusca are formed by a 

 secretion from glands of the mantle or cloak. In uni- 

 valves this part of the body only covers the front, and in 

 most cases surrounds the head like a loose collar ; but 

 it is very flexible, and it can be withdrawn or folded 

 back nearly to the top of the spire to repair a fracture 

 of the shell in that part. In bivalves it is double, like 

 the cover of a book. 



Composition. — Carbonate of lime is the main ingre- 

 dient ; and the sheUs of Mollusca difi'er from the bones 

 of vertebrate animals, as well as from the shells of crabs, 

 sea-eggs, and birds^ eggs, in the absence of phosphate of 

 lime. In all these cases, however, the mineral ingre- 

 dients are cemented together by an animal gluten. 

 According to M. Delacroix, the shell of a Helix pomatia 



