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20 A77ierjca?i SeasheUs
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