346 MARINE INVERTEBRATES 
outer covering of the shell is excessively thin. In old speci- 
mens the epidermis is usually worn away from the apical whorls. 
It often happens that the shells of the most exquisite colors and 
markings are thus rendered somber and unattractive by their 
tenants during life; when the animal dies, or when such speci- 
mens are kept in a cabinet for a period of time, the epidermis 
dries, cracks, and falls off, revealing the wealth of color and 
design beneath. 
The growth of the gasteropod shell is accomplished by the 
exudation from the margin of the animal’s mantle of a liquid 
containing the shelly matter in solution. The mantle-edge is 
provided with a complicated system of glands and pores, from 
which is secreted this “stony liquor.” This more or less viscous 
liquid, containing the carbonate of lime and the other inorganic 
materials of which the shell is composed, hardens upon exposure, 
and the shelly matter is then deposited in crystalline form around 
the edges or lip of the shell aperture. The gasteropod shell 
therefore grows by the continual building out of its aperture 
through successive depositions of shelly matter at the extreme 
edge of the lip. At the extreme edge of the mantle margin are 
situated those glands which secrete the materials tor the epider- 
mis of the shell, and as one would therefore expect, this outer- 
most layer of epidermis is first produced in the advancing growth 
of the shell. There also are situated the pigment-glands, which 
produce the color-secretions. The various layers of the shelly 
substance are successively deposited inside the mouth of the shell 
by glands situated just back of the extreme edge of the mantle 
margin. Thus in the growth stage, if one could examine closely 
the aperture of a gasteropod shell, one would observe at the ex- 
treme tip of the lip this projecting epidermis, just beneath it and 
just inside the aperture a thin deposit of shelly matter, just be- 
neath this, and farther in, another layer, and still farther in a 
third layer. 
The growth of nearly all gasteropod shells is marked by periods 
of rest. During the inactive seasons the creature may thicken 
the edge of the aperture to a greater or less extent by an extra 
deposit of shelly matter, for otherwise the thin lip might soon be 
