CRUSTACEA IN RELATION TO MAN 253 



species of the Copepod Cyclops (see Fig. 14, p. 39). 

 In some way not yet understood, the embryos 

 penetrate into the body cavity of the Cyclops, where 

 they undergo a metamorphosis. For their further 

 development it is necessary that the Cyclops should 

 be swallowed by man, as may easily happen in 

 drinking water from a pond. When the Cyclops is 

 digested the larval worms are set free, and they bore 

 their way through the tissues of their human host 

 till they reach the place (generally under the skin of 

 the leg) where they complete their development and 

 produce the innumerable embryos that are set free 

 in the way just described. 



A few Crustacea inflict a certain amount of injury 

 on man in more indirect ways. In tropical countries, 

 Land Crabs are often troublesome in gardens, and 

 may cause serious damage to young plants in sugar- 

 cane plantations and rice-fields. In gardens in this 

 country, the Woodlice, as already mentioned, are 

 sometimes destructive to seedlings and delicate 

 plants. The little fresh-water Isopod, Asellus aquati- 

 cus, is accused of destroying the nets used in fishing 

 for Pollan in Lough Neagh in Ireland. 



Probably the most important of all Crustacea, 

 however, as regards their destructive activity, are the 

 species which bore into wood, and sometimes do exten- 

 sive damage to the submerged timber of piers, jetties, 

 and similar structures. On our own coast the most 

 destructive is a little Isopod known as the " Gribble " 



