GENERAL INTRODUCTORY 3 



locomotion, takes various forms in the different 

 groups of Mollusca ; the Univalves (Snails and 

 Whelks) creep along by its means ; the Bivalves 

 employ it to burrow with ; in the Cuttlefish it is 

 drawn out into the " arms " ; in other Mollusca it is 

 transformed into fins to swim with ; whilst in some, 

 like the Oyster, it has ceased to be used, and has 

 degenerated into a mere rudiment. Most of the 

 muscles of the body are concerned with the extension 

 or retraction of the different organs of the body, and 

 do not call for special enumeration. 



The shell is mainly composed of carbonate of lime, 

 as much as 95 per cent., in the form of calcite or 

 arragonite, being often present, with the admixture 

 of a chitinous substance, " conchyolin " ; a little 

 phosphate of lime and a trace of carbonate of mag- 

 nesium are also present. It originates in a shell- 

 gland, or pit, in the embryo, and the successive 

 layers of which it is built up (Plate VII., Figs. 11 

 and 12) are formed as the animal grows by additions 

 to the margin, and are deposited in order from the 

 outermost to the inner one by a series of special 

 cells situated in the thickened margin of the mantle. 

 The outermost layer, or "periostracum,"* contains 

 the greatest abundance of chitine-like material in its 

 composition, and is the work of the cells at the very 



* The term " epidermis," which has been extensively mis- 

 applied to this layer, should be reserved exclusively for the 

 outer layer of the skin of the animal itself. 



