124 THE LIFE OF THE MOLLUSC A 



well as the sponges, needle-like spicules which make 

 them undesirable as articles of food ; hence the im- 

 portance of the mimicry. 



How far one mollusc devoid of special self-defence 

 may mimic another kind furnished with some de- 

 fensive protection is a moot point. It has been 

 suggested that a certain species of Strombus, the 

 shape of whose shell resembles that of a Conns, may 

 derive some advantage thereby in view of the 

 poisonous bite of the latter. 



There seems more probability in the case of two 

 Philippine Land Snails — Helicarion tigrinus and Xesta 

 Cumingii — both possessed of a small thin shell, into 

 which the animal cannot retire, and a remarkably 

 long, narrow, high-ridged " tail " to the foot, which 

 forms the most likely looking hand-hold for their 

 capture. When so seized the Helicarion parts with 

 its tail and escapes, while the Xesta is unable to do 

 so, and it is thought that birds acquainted with the 

 futility of attempting to capture the one would leave 

 the other in peace also. 



The alimentary system in its entirety does not 

 present features of evolutionary interest, although 

 minor variations in response to individual require- 

 ments are numerous. Of these the most interesting 

 is the development in the gizzard of the Bullidse of 

 calcareous plates (Plate III., Figs. 9 and 10), whose 

 function is to crush the small shellfish, which are 

 swallowed entire. 



