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 for 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 



