CRUSTACEA AND MAN 1 63 



back. The young, which emerge from the female brood pouch as 

 miniature adults, start their own burrows from the side of the 

 parental burrow. Each female may produce three broods of twenty 

 or thirty young in a season, so that extensive damage can be 

 inflicted in a couple of seasons. Attacks are generally most severe 

 at low water mark, and the damaged piles often taper towards their 

 bases, like well-sharpened pencils. 



An amphipod, Chelura terebrans, is also found in burrows in piling, 

 but it does not seem to be a very good borer on its own, and usually 

 relies on the activities of the gribble to provide it with a home. 



Another crustacean activity detrimental to man is the transmis- 

 sion of diseases. In general the Crustacea are not very important 

 from this point of view, but in the tropics they play a part in the 

 life cycles of some objectionable parasites. One of these is the 

 guinea worm, Dracunculus medinensis, which is a large nema- 

 tode, or roundworm, reaching a length of three or four feet. The 

 adults live in the tissues just under the skin of man, and the females 

 liberate living young through ulcers, usually on the legs of the host. 

 The young are liberated when the host limbs are in water, and they 

 can swim quite actively. Further development occurs when the 

 small worms are swallowed by a copepod, Cyclops. After three weeks 

 in Cyclops the worms are then in an infective state and when 

 Cyclops is swallowed by man with his drinking water the cycle is 

 completed. 



The largest of the tapeworms which infect man also has an inter- 

 mediate stage in a copepod. Diphyllobothrium latum, the broad 

 tapeworm, reaches a length of sixty feet. Man becomes infected by 

 eating raw or insufficiently cooked fish, and the fish becomes 

 infected either by swallowing an infected Cyclops, or by eating 

 another fish that has eaten an infected Cyclops. In this case the 

 copepod is not the direct transmitter to man, but is once or twice 

 removed down the line of intermediate hosts. A more direct trans- 

 mission of a parasitic worm is found in certain river crabs. In 

 various parts of the old world these crabs are infected with the 

 larval stages of a fluke, or flatworm, of the genus Paragonimus. 

 These live as adults in the lungs of man, and the eggs are ejected 

 with sputum. A larval stage emerges from the e^g and infects a 

 snail, eventually giving rise to a different type of larva. These larvae 

 infect river crabs and crayfish, and the infection passes to man 

 when these Crustacea are eaten raw. The young worms make their 

 way through the wall of the gut and migrate up into the lungs. 



M* 



