20 



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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 pro- 

 tective, horny periostracum. This is a tough, mat-like and often hairy cover- 

 ing to the shell which prevents acids and marine growths from doing damage. 



Figure 5. Four stages in the growth of the Keyhole Limpet, Fissiirella, 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. Peli- 

 can'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 thick- 

 ened rib may be formed at the edge of the shell lip at regular intervals regard- 

 less 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 



