18 THE ESSEX NATURALIST
rises and swims with the shell aperture in front. Yet another
genus (Lima, Pl. 1) swims with the shell valves in a vertical
position, the slow movements and the many bright trailing
filaments producing a very beautiful effect.
Fig. 5. Diagram of Pecten, the scallop swimming. 1m, left (upper) lobe
of mantle: lv, left (flat) valve of shell: rm, right lobe of mantle: rv, right
(rounded) valve of shell. Continuous arrows show directions of water jets
forced out by closure of shell. Broken arrow shows direction of swimming.
Returning to the gastropods, similar types of adaptation for
different habitats have occurred in this class, but as they are
often more active than the bivalves their habits are sometimes
more spectacular. They also include a number of genera which
are carnivorous.
The more primitive members (Diodora, Gibbula, Pl. 1)
are largely confined to rocky shores although the limpets, a
hardier race, can be found wherever there is a firm foothold.
The winkles (Littorina) with a more efficient gill and better
cleansing arrangements for the mantle cavity are found on all
shores.
Many gastropods burrow in sand or sandy mud (Turritella,
Aporrhais, Pl. 1) selecting their food from the particles in
the inhalent water stream which they create. A number
(Crepidula, Pl. 1) are filter feeders using the single gill in a
fashion analogous to that of the bivalves. Crepidula, the
slipper limpet, is a pest on the oyster beds.