18 



ing into the tubes of its fellows, or into otlier spaces, 

 if its course can bo no longer changed, it contracts the 

 anterior part of the body slightly, secretes a closed cal- 

 carooiis lining in front of it, and ceases to burrow furth- 

 er and to grow. Otherwise, it seems it may grow indefi- 

 nitely, and it is difficult to predict how large specimens 

 of T. norvegica might become, were there not adverse con- 

 ditions to stop tliGir growth. 



I believe the calcareous lining of the burrow has 

 been acquired primarily for the protection of the very 

 delicate body from the rough surfaces of the wood. But it 

 serves other purposes also. It prevents the diffusion of 

 injurious substances into the burrow, and also prevents 

 the intrusion of other creatures that live in the wood. 

 Then, too, v/hen the surrounding wood decays, or is eaten 

 av;ay by other animals, so as to endanger the life of the 

 "Ship-v.'orm" , the tube may be so strengthened as to serve 

 as the sole means of external protection. In this way the 

 walls of tubes which, protected, are usually not over a 

 qviarter to a half millimeter thick, may become two milli- 

 meters or more thick. This response of the animal to 

 changed conditions on the outside is a very mysterious one 



