ZOOLOGY. 419 



of soft flesh, but is provided with bone-like cutters, or teeth, 

 with which it bores through hard wood, sometimes making a 

 hole a third of an inch in diameter. It usually follows the 

 grain, lives only in wood below high tide in salt water, and 

 never descends far below low tide. The mixture of fresh 

 water with that from the sea diminishes the activity of the 

 teredo, and in seasons of drought they do comparatively much 

 injury in San Francisco Bay, but little after abundant rains, 

 continuing late into the summer. The eggs are thrown out 

 upon the water and carried about by the current. If they 

 stick upon wood, they hatch and bore in, and once inside they 

 never leave it till it is converted into honey-comb. Piles fif- 

 teen inches through, unless covered with metal or filled with 

 some substance (creosote, for instance) offensive to the ship- 

 worm, are usually rendered worthless in five years, and some- 

 times in three. 



Another harbor pest is the gribble, (Limnoria) a worm 

 about one-tenth of an inch long, which lives in wood exposed 

 to sea water, between high and low tide, and unlike the teredo, 

 eats across the grain, and comes out to the surface. 



