16 



lience wore little ii" any over a year old. 



The rate of growth seems to depend but little if at 

 all, on the hardness or kind of wood. It is well knovm , 

 "Ship-worms" penetrate all kinds of wood, whether it be 

 soft white line or hard oak. In India there are types 

 that bore into stiff clay. None of our species adopt such 

 a habitat, so far as I know. But I have found small, ab- 

 normal specimens of Xylotrj''a in very rotten wood, and I 

 take it that their' abnormal character v/as due to unusual con- 

 ditions. In this case they v/ere associated with Xylor'haf'a 

 dorsalis and Pholas dactylus. However, in wood proper, I 

 have observed that they grow quite as rapidly in hard yel- 

 lo.Y pine as in soft white pine; so that the rate of growth 

 seems conditioned by food supply, and not by the ability 

 of the animal to form its burrow. 



Protective Adaptations. - The life of the "Ship-worm" in 

 the wood has led to profound changes in tiie character of 

 its external parts and its means of protection. As the 

 "V/orm" enters, the posterior part of the body projects 

 more and more beyond the shell v/hich loses its protective 



