12 THE ESSEX NATURALIST
An Introduction to the Mollusca
By E. H. NISBET, Ph.D., B.Sc.
In making this short survey of the Phylum Mollusca it is
important to bear in mind that in order to survive, all
animals must be able to perform a number of basic functions.
They must be able to recognise and to obtain food, which they
must be able to digest and assimilate: to obtain food they must
be able to move : to move, they need to utilise the energy in the
assimilated food, a process which we call respiration. This will
give rise to waste materials which must be removed and this
we call excretion. A new-born animal grows until it becomes
an adult and then, in order that the race may survive, it must
reproduce its kind.
In each of the great divisions of the animal kingdom the
body of the individual is composed, in greater or lesser degree,
of organs which sub-serve these basic functions. But in each
of these divisions or Phyla a limit is set to the size of its
members, to the complexity of their structure, and to the level
of activity to which they can aspire. This limit is determined on
the one hand by their genetic inheritance, which dictates both
the general form of their bodies and the types of variation
which may be possible: and on the other by the physical factors
of the environment, eg., temperature, the density of the
external medium, desiccation.
Thus the Coelenterata, a phylum to which the sea-anemones
belong, are sac-like creatures lacking special systems of organs.
Food, digested in a large central cavity, reaches all parts of the
body by diffusion. Respiratory and excretory processes are
similarly dealt with, while their movements are controlled by a
relatively inefficient nerve net and by slowly contracting muscle
fibres. Above the level of the Coelenterates four body plans
have been markedly successful. These are (a) the round-worm
plan of the Nematodes, simple but highly efficient, (b) the
segmented Arthropod plan, the efficiency of which is demon-
strated by the swarms of Crustacea in the seas and the still
greater swarms of insects in the air, (c) the Vertebrate plan,
also segmented, to which we ourselves adhere and hence tend
to regard as especially successful: and (d) the unsegmented