146 THE SEAS 



do this by hollowing out little burrows from the side of 

 the parent burrow, and never by leaving the burrow and 

 beginning a new one in fresh timber. As we shall see later, 

 the fact that fresh timber is always infected by adult and 

 not young Gribbles is unfortunate from the point of view 

 of the protection of wood. 



The ravages of the Gribble are always clearly apparent on 

 the surface of the wood which is gradually rotted away until 

 the outer layer falls off ; the Gribble is then able to penetrate 

 still deeper and so, layer on layer, the wood is destroyed 

 (Plate 52). The Gribble is always especially abundant 

 in such structures as pier piles about low-water mark and 

 here it eats deepest into the wood, which tapers away and 

 finally breaks through at this point. So numerous are 

 they in badly infected wood, that between 300 and 400 

 Gribbles have been collected from a square inch of timber. 



There is no evidence that the Gribble can actually 

 digest the wood though it certainly swallows large quantities 

 of the fragments which are bitten off by its powerful 

 mandibles. It has been found boring in the insulating 

 covering of submarine cables so that clearly it does not 

 depend on wood to the same extent as does the Shipworm 

 which has never been found anywhere but in wood. Some 

 of the other crustacean borers and the mollusc Xylophaga 

 have also been found in the insulation of cables and all 

 these creatures appear to bore for the protection it affords 

 them and not for the purpose of obtaining food. 



There are several other crustacean wood borers. The 

 most common of these is a creature known as Chelura 

 terebrans (it has no common name, unfortunately) which 

 is slightly larger than the Gribble, and flattened from side 

 to side, being a relative of the common sand-hoppers of 

 the shore. It usually works along with the Gribble, 

 but nearer the surface of the wood, and is almost as world- 



