826 NATUEAL HISTOET OP AQUATIC ANIMALS. 



considerably larger and more oblique in their direction, so that the surface of the timber thns 

 undermined by these destructive animal* is rapidly washed away by the action of the sea, ani. 

 the excavations are exposed in the greater part of their extent, the wood appearing plowed 

 up, so to speak, rather th^an burrowed into. ' Upon the whole, Ghelura would seem to be a still 

 more destructive creature than even lAmnoriaP 



2?9. THE ISOPODS. 

 Ths Geibble, or Boring Limnoria — Limnoria lignorxjm, White. 



This little crustacean pest, which measures less than one-flfth of an inch in length, is a very 

 common habitant of our Atlantic coast from the G-ulf of Saint Lawrence to Florida, and also 

 occurs abundantly on the coasts of Great Britain and of other parts of Europe. In spite of its 

 smaU size, it is very destructive to all kinds of submarine wood-work, which it rapidly eats away. 

 Its body, which is subcylindrical in shape, consists of fourteen segments, the anterior one being 

 the head ; the two ends are rounded and the sides are nearly straight and parallel to one another. 

 The first seven segments, not including the head, bear each a pair of short legs. It makes its 

 burrows by means of stout mandibles or jaws. In color it is grayish, the upper surface of the 

 body being covered with minute hairs, tq which more or less dirt usually adheres. 



The Gribble generally lives above and just below low-water mark, but has been found at 

 times, though very rarely, as low down as seven to ten fathoms. It gnaws burrows into all sorts 

 of sunken or floating wood near the shore, and lumber or drift-wood left for some time on muddy 

 shores is pretty certain to be attacked by it. The burrows are made to a depth of about half an inch, 

 and when they become numerous enough to reduce the superficial layer of wood to a mere honey- 

 comb, it scales off, leaving a fresh surface, which is at once attacked. Much damage is done by 

 this Uttle creature to the piles of wharves and other submarine wood-work all along our Atlantic 

 coast, and numerous methods of stopping its ravages have been devised. It has been observed 

 attacking the gutta percha of submarine telegraph cables. Professor Verrill describes its habits 

 and the damage it has done on the American coast, as follows : ^ 



"It has the habit of eating burrows for itself into solid wood to the depth of about half an 

 inch. These burrows are nearly round, and of all sizes up to about a sixteenth o* an inch in 

 diameter, and they go into the wood at all angles and are usually more or less crooked. They are 

 often so numerous as to reduce the wood to mere series of thin partitions between the holes. In 

 this state the wood rapidly decays, or is washed away by the waVes, and etery new surface 

 exposed is immediately attacked, so that layer after layer is rapidly removed, and the timber thus 

 wastes away and is entirely destroyed in a few years. It destroys soft woods more rapidly than 

 hard ones, but all kinds are attacked except teak. It works chiefly in the softer parts of the wood, 

 between the hard, annual layers, and avoids the knots and lines of hard fiber connected with them, 

 as well as rusted portions around nails that have been driven in, and consequently, as the 

 timbers waste away under its attacks, these harder portions stand out in bold relief. Where 

 abundant it will destroy soft timber at the rate of half an inch or more every year, thus dimin- 

 ishing the effective diameter of piles about an inch annually. Generally, however, the amount is 

 probably not more than half this, but even at that rate' the largest timbers will soon be destroyed, 

 especially when, as often happens, the Teredos are aiding in this work of destruction. It lives in 

 a pretty narrow zone, extending a short distance above and below low- water mark. It occurs all 



'Vineyard Sound Report, pp. 379, 380, 1871-'72. 



