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. 
