144 THE SEAS 



as though the animals must come out of their burrows as 

 they increase in size and start new ones. Another way in 

 which the Gribble differs from the Shipworm is that it is 

 always the fully-grown animals which start new burrows. 

 To do this, they first of all hollow out a groove along the 

 surface, keeping in the soft part of the grain, and then 

 pass by a very easy incline into the wood. Usually the 



Fig. 31. — Mandibles of the Gribble, Limnoria, showing " rasp-and-file " 

 combination, greatly enlarged (after Hoek). 



burrows are not deep ; the need for obtaining a constant 

 supply of fresh sea water probably controls this, and, 

 though they have been found as deep as three-fifths of an 

 inch below the surface, they do not usually penetrate for 

 more than a third of this distance. The burrows are 

 often three-quarters of an inch or more in length. It 

 is easy to follow the course of a burrow from above be- 



