76 ANIMAL FORMS 
the bottom of its burrow, it extends its slender siphons up 
the tube and out of the entrance for its food supply. 
Often hundreds of individuals enter the same piece of 
wood, which becomes thoroughly riddled within a short 
Fig. 46.—The piddock (Zirphea crispata), a rock-boring mollusk. Natural size, 
from life. 
time, and though giving no outward sign of weakness may 
collapse with its own weight. Incalculable damage is thus 
rendered to the shipping interests, and in consequence 
much has been done to check their ravages, but they are 
far from being completely overcome. 
76. Other stationary species.—A large number of other 
species, while small and inconspicuous, are also free to 
