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 (Cyprcea), 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 infra, p. 105). 



The muscular foot, which is generally an organ of 



