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.
Fig. 80—The Gribble (Limnoria lignorum). Much enlarged (From British Museum Guide, after Sars.)
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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.
Probably the most important of all Crustacea, however, as regards their destructive activity, are the species which bore into wood, and sometimes do extensive 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" (Limnoria lignorum—[Fig. 80]), which is distributed from Norway to the Black Sea, and occurs also on the Atlantic coast of North America. Several species of the same genus having similar habits are found in other parts of the world. The Gribble was first discovered as a British species by Robert Stevenson, the celebrated lighthouse engineer, who found it in 1811 destroying the woodwork employed in the erection of the Bell Rock Lighthouse, and sent specimens to Dr. Leach of the British Museum. The animal is only about one-eighth of an inch in length, and its cylindrical burrow is about one-fifteenth of an inch in diameter, and penetrates for a depth of one or two inches. The excavation of the wood is effected by means of the mandibles, which are unusually strong; and when the animals are numerous the burrows are driven so close together that the surface of the wood is reduced to a spongy mass which is rapidly washed away by the waves ([Plate XXXII].). The Gribble is often accompanied by another Crustacean of similar habits, the Amphipod Chelura terebrans. The latter is about one-fifth of an inch in length, and differs from most Amphipods in having the body somewhat flattened from above downwards instead of from side to side. The burrows made by Chelura are shallower than those of the Gribble, and generally run more or less parallel to the surface of the wood.