AN INTRODUCTION TO THE MOLLUSCA 17
composed of microscopic plants and animals, it is not surprising
that the gills come to be used as food-collecting organs. This
has happened many times in different groups of molluscs but
in particular, practically all the bivalve molluscs have developed
this habit and in so doing, have undergone a reduction of the
head. Nothing is left of this structure except a mouth with
long grooved lips or palps and the cerebral ganglia of the
nervous system.
Within the limits imposed by their feeding methods the bi-
valves have been able to colonise many aquatic habitats. We
have all seen the banks of edible mussels (Mytilus edulis) on
our coasts, wherever there is a firm object to which they can
attach themselves. Perhaps few are aware that they can move
slowly about by removing their attaching threads and forming
fresh ones in other positions as they move. Members of this
Club are familiar with the edible oyster (Ostrea edulis) which
attaches itself to a stone or another shell when it settles, but
can live freely on the surface of the mud as it grows provided
that there is no violent wave action. Other bivalves partly
bury themselves in sand (Paphia (Pl. 2), Glycymeris and
our fresh water Unio and Anodonta) while yet others bury
themselves completely in sand (Cardium, Pl. 3, Ensis, Pl.
2), muddy sand (Mya, Lutraria, Pl. 3, Tellina) or in mud
(Scrobicularia, Pl. 2). In these genera long inhalent and
exhalent siphons are formed from the mantle so that they may
respire and feed while remaining hidden beneath the sand or
mud. Yet another group of bivalves is adapted to burrow into
harder materials such as chalk and limestone (e.g. Pholas,
Pl. 1) using the hard and serrated valves of the shell to do so.
Such animals are important causes of erosion of cliffs on the
shore. Others such as Teredo (the ship worm) burrow into
wood, causing serious damage to piles and ships' timbers in
many parts of the world.
One of the most interesting adaptations in bivalves is that
which enables some of them to swim. For example, the
scallop (Pecten, Pig. 5 and Pl. 3) and Chlamys (the
Queen) have formed curtains from the mantle which prevent
water from passing out of the animal except as directed
streams. Rapid opening and closing of the shell valves causes
two jets of water to pass outwards, one on each side of the
hinge, and a weaker stream downwards so that the animal