AN INTRODUCTION TO THE MOLLUSCA 19
Some are scavengers and carrion eaters (Buccinum,
Nassarius, Pl. 1) : others (Ocenebra the sting winkle and
Nucella the dog whelk, Pl. 1) are carnivores boring holes in
the shells of other molluscs and scooping out the soft contents.
These gastropods also possess an inhalent siphon through
which incoming water impinges upon a sensory area within the
mantle cavity. This arrangement appears to function as a
"nose" by means of which they find the carrion or the prey.
Another interesting group are burrowing carnivores
(Scaphander, Pl. 2, Philine). Their shells have been reduced
in size and they have developed a muscular covering to the
head. Ploughing their way through the sand they search for
small bivalves, swallowing them whole and crushing them in a
powerful gizzard. Pleurobranchus (Pl. 2) with a much
reduced shell, is somewhat similar in its habits.
A further carnivorous group is that of the "sea slugs" or
Nudibranchiata, but these have very different habits.
Archidoris (Pl. 2) brightly coloured and distasteful to
other animals, feeds on sponges. AEolidia (Pl. 2) feeds on
sea anemones and uses the stinging cells of its food as its own
defensive weapons, passing them into the soft dorsal processes
on its back where they can be exposed to daunt any attacker.
Of the many swimming gastropods but one is shown.
Aplysia, the Sea Hare (Pl. 3) is an herbivore with lateral
expansions of the body which serve as finis. It crawls on the
shallow sea floor, feeding upon Algae, but at intervals swims
away to other pastures, looking remarkably like a skate as it
undulates its "fins."
The last important order of gastropod molluscs is the Pul-
monata. These animals have lost their gill and have turned
the mantle cavity into a "lung" for air breathing. A few are
found on the sea shore (Onchidella, Pl. 1), more are found
in fresh water (e.g. Limnaea, Planorbis) where they come to
the surface at intervals to replenish the air in the mantle
cavity. Many are terrestrial, either burrowing sporadically
(Helix, Pl. 1) or burrowing consistently during the day and
coming out to feed at night (slugs, e.g., Limax).
Perhaps the most spectacular class of molluscs is that of the
Cephalopoda (octopuses and squids, Pl. 3). These animals