2 THE LIFE OF THE MOLLUSCA 
are familiar examples, as now restricted, form a well- 
marked group (subkingdom or phylum) of the animal 
kingdom. 
Owing to their plasticity, they differ very much 
among themselves in external form ; indeed, some of 
the more aberrant members are at first scarcely recog- 
nizable as molluscs at all; nevertheless, apart from 
the protective shell, which is a leading feature of the 
group, a remarkable uniformity characterizes their 
internal organization in its main features, especially 
in the young forms. : 
Externally most possess a “head”; a ventral 
creeping organ, the “foot”; and a dorsal covering, 
the “mantle,” which bears and secretes the shell. 
This shell forms a protection to the more vital 
organs, and into it the animal can generally with- 
draw for security from attack. 
The mantle does not usually reach far beyond the 
shell-margin when the animal is extended, but in 
some cases it curls round over the shell (Plate II, 
Fig. 2), and even, as in the Cowry (Cypr@a), meets 
on the top (Plate II., Fig. 9). In the more special- 
ized Mollusca there is a tendency to the reduction, 
even to disappearance, of the shell, and in these 
there is a corresponding liability for the shell to 
become more and more enveloped permanently in 
the mantle as the animal becomes less and less able 
to use it as a place of retreat (see fra, p. 105). 
The muscular foot, which is generally an organ of 
