BORING LIFE 139 



the hinder muscle, the frontal portions of the shell are drawn 

 apart so that the sharp ridges with which they are covered 

 scrape off the surface of the wood while at the same time 

 the ridges on the middle lobe widen the opening behind. 

 The small muscle between the frontal lobes now comes into 

 play and by contracting in its turn brings the frontal lobes 

 together again, which is a much easier operation because 

 the surface of the wood offers no resistance to movement 

 in this direction. The foot then loosens its hold and moves 

 a short distance to one side, when the same process is re- 

 peated. This goes on time after time until the shell and 

 front half of the body have twisted completely round — 

 the hinder end cannot do this because it is attached to the 

 burrow in the region of the pallets — when the movement 

 is reversed. As a result of this continuous working of the 

 shell and twisting of the front end of the body first in this 

 direction and then in that, the inside of the burrow becomes 

 perfectly smooth and circular in cross section. Immediately 

 behind the shell the surface of the wood becomes covered 

 with a layer of shelly substance which the animal produces 

 in the naked region of its body. The whole process of 

 boring is a model of mechanical efficiency which is not to 

 be surpassed by any member of the animal kingdom. 



The mouth of the Ship worm lies just above the sucker- 

 like foot. Into it pass, not only the tiny particles collected 

 from the sea water which enters through the siphons, but 

 also the minute fragments of wood which are cut away by 

 the boring organ. All of these have to pass through the 

 gut of the animal before they can be discharged into the 

 sea water by way of the second of the siphons. It has 

 long been a matter of dispute whether the Shipworm can 

 actually digest the wood or whether this passes through the 

 body unchanged, but recent investigations seem to show 

 quite definitely that the animal does obtain a considerable 



