98 SEA-SHORE LIFE 



sort of wood excepting teak. It burrows tlirougli tlie softer parts of 

 the wood, completely lioney-coinbing the logs so that the knots and 

 hard parts project. Submerged timber decreases in diameter at the 

 rate of about an inch per year due to the ravages of this pest. Fortu- 

 nately, however, it does not occur in any considerable numbers much 

 below tide level. Submerged wood must be sheathed in copper or 

 frequently painted with creosote or poisonous substances, in order 

 to prevent its destruction. 



CRABS 



Brachyura. 



The crabs are the highest of the Crnstacea. The head and 

 middle ])art of the body are covered with a shell which is usually 

 broader than long. The abdomen is small, and is folded forward, 

 fitting into a groove on the lower surface of the animal. In the 

 male the abdomen is narrow, and has only two pairs of append- 

 ages, while in the female it is much broader, and has four pairs of 

 appendages which serve to carry the eggs (see Fig. 47, Page 79). 

 The two pairs of feelers are small, and the eyes are mounted on 

 movable stalks which may usually be folded back into appropriate 

 sockets. The mouth appendages are broad and flat so as to 

 cover the opening of the mouth itself. The first pair of legs 

 have pincers, while the following four pairs lack pincers but are 

 formed for walking or clinging; the hindermost being often paddle- 

 shaped and used in swimming. The plumed gills are contained in 

 side chambers enclosed by the shell of the middle part of the body. 

 Being thus protected from drying, many of the crabs may live for 

 long periods of time on land, their gills remaining moist. Crabs 

 live in both salt and fresh water, and at all depths. Some are active 

 swimmers or crawlers while others are sedentar}^ or even parasitic. 

 They are scavengers, devouring almost any dead and decaying 

 animal or plant. Despite their imclean habits, many species are 

 highly esteemed as food, and crab fisheries are of great commer- 

 cial importance. When hatched from the egg, crabs are totally un- 

 like their adult form, and are said to be in the Zoceci stage. The 

 head and middle part of the body are covered with a delicate shell 

 which gives rise to four long, sharp spines. One of these is directed 



