AN INTRODUCTION TO THE MOLLUSCA 13
Molluscan plan, perhaps less successful than the foregoing but
sufficiently versatile to have populated the seas, fresh water
and the land with about 80,000 species divided into six classes.
Fig 1. Dendrocoelum lacteum, a white Planarian very common in our pools
and streams, e, eye: g, branching gut: p, position of mouth and pharynx.
Natural size indicated.
The Mollusca are a very old and distinctive group of animals.
Their shells are found in Cambrian strata and they appear
already to have evolved their existing classes at that time.
Thus during a period of more than 500 million years, while
the vertebrate classes were coming into existence, the molluscs
have held their own. Although most are slow-moving animals
and none is ever likely to acquire wings or to colonise deserts,
their organisation can be said to have stood the test of time.
We cannot see enough of the past to be at all sure of the
origins of the Mollusca, but perhaps we should not be far wrong
if we thought of them as having been derived from animals
rather like those lowly flatworms—the Turbellaria or Plan-
arians (Fig. 1)—found in the sea and in our streams.
These worms are flat because they have no blood system and
hence must spread the gut to all parts of the body, and also
keep all parts of the body close to the surface in order that their
tissues may be close to the oxygen of the external medium.
With the development of a blood system to transport food and
oxygen, and with gills to collect the latter, the animal may be-
come, larger, develop a more complex and efficient digestive
system and also provide itself with a protective covering in
the form of a shell.