AN INTRODUCTION TO THE MOLLUSCA 15
the entrance to a cavity into which clean water must flow for
respiratory purposes and out of which excretory matter and
faeces must pass : (c) many of the more advanced gastropods
untwist as they become adult, although they show the twisting
as larvae : (d) the larvae of marine gastropods start life in a
symmetrical condition and then during their development sud-
denly undergo this characteristic torsion.
Fig. 3. Diagrams of a veliger larva (a) before and (b) after torsion. a,
anus: f, foot: gu, gut: h, head: m, mouth: mc, mantle cavity: o, operculum:
sh, shell: v, velum—the cilia of which enable the larva to swim.
Much magnified.
If torsion is such a nuisance to the adult gastropod why should
it happen at all? To this question W. Garstang (1928) seems
to have given the best answer. One important function of the
mantle cavity (especially in the larvae) is to provide a safe
refuge into which the animal can withdraw if attacked. If the
mantle cavity is situated posteriorly the horny foot is retracted
but the delicate head is left exposed. Bringing the cavity to
the front reverses this position and allows the head to be
retracted first, the door then being closed by the horny oper-
culum (Pig. 3). This is an instance in which larval
adaptation has profoundly influenced adult structure. The
additional coiling seen in many gastropod shells has the
advantage of making the body more compact but adds to the
complexity of the picture.
Starting from the condition of the primitive mollusc pictured
above, compelled to live on rocky shores in clean water and
feeding upon microscopic algae with their toothed ribbon-like