shell. This covering mantle continually adds thin paintings of shell material over the entire outer surface of the adult shell. In these groups, where the outside of the shell is protected by the mantle, there is no production of protective, horny periostracum. This is a tough, mat-like and often hairy covering to the shell which prevents acids and marine growths from doing damage.
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Figure 5. Four stages in the growth of the Keyhole Limpet, Fissurella, showing how the spire gradually disappears and the marginal slit becomes an apical hole. a, b, c, highly magnified; d, natural size. (After Boutan, 1886.)
In contrast to the bivalves, many gastropods exhibit certain modified shell structures when they become reproductively mature. In many instances adulthood of an individual is accompanied by the formation of a thickened or flaring apertural lip. It is most pronounced in the Strombus conchs. Pelican’s Foot (Aporrhais) , marginellas, cowries and others. Such development is unknown in the cone shells, Busycon whelks, abalones and a host of others.
Growth of the gastropod shell is more rapid in young individuals. Some species apparently continue to grow in size during their entire life span, while others cease once sexual maturity is reached. In the murex shells and frog shells (Cymatium) and certain Cassis helmet shells a strong varix or thickened rib may be formed at the edge of the shell lip at regular intervals regardless of sexual or seasonal conditions. Each thick varix represents a resting period in growth. Collectors may have noticed that they seldom find murex shells in a growth stage between varices. This is because over ninety percent of the snail’s life is spent in the varix stage and because additional growth between varices takes place in less than two days.
The color pattern of shells is a graphic representation in time of the secretory activity of the pigment-producing cells located along the mantle edge. The ground color is produced by the whole line of cells; banding is produced by the special activity of groups of cells, often sharply localized. Where the activity of these groups is cyclical, blotching results; where the active focus moves up and down the mantle edge, or where activity spreads from a focus, there may be formed zigzag, V-shaped or circle patterns. A
