matter of the richness or poverty of the plankton harvest. None of the Crustacea can be regarded as directly harmful to man. They have not the power of inflicting envenomed wounds which renders some other Arthropods, such as Scorpions, some Spiders, Centipedes, and Insects, formidable in spite of their small size; and although blood-curdling tales of the ferocity of the Land Crabs are to be found in the accounts of old voyages, even the largest of these is hardly an antagonist to be dreaded. A considerable number of invertebrate animals, not of themselves noxious, are now known to be the indirect cause of much serious injury to human life by harbouring and disseminating organisms which produce disease. The progress of research is adding, almost every day, to the number of species known to be disease-carriers, and it is possible that in the future some Crustacea as yet unsuspected may be added to the list. At present, however, there is only one case in which a Crustacean has been shown to be concerned in the transmission of a parasite of man. The "Guinea-worm," _Filaria_ (or _Dracunculus_) _medinensis_, is a parasite belonging to the group of "Thread-worms" (Nematoda) which causes dangerous abscesses under the skin of the legs in many parts of tropical Africa. It has been shown that the embryos of the worm, which are discharged in vast numbers on the bursting of the abscess, do not develop unless they fall into water containing certain 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 aquaticus_, is accused of destroying the nets used in fishing for Pollan in Lough Neagh in Ireland. [Illustration: FIG. 80--THE GRIBBLE (_Limnoria lignorum_). MUCH ENLARGED (From British Museum Guide, after Sars.)]