124 THE LIFE OF THE MOLLUSCA 
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 Stvombus, the 
shape of whose shell resembles that of a Conus, 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 
Cumingti—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 Bullide of 
calcareous plates (Plate III., Figs. g and 10), whose 
function is to crush the small shellfish, which are 
swallowed entire. 
