CRUSTACEANS In our inland counties the rude forefathers of the hamlet were con- tent with a classification of the animal kingdom in which crustaceans had no share. Apart from a few birds and mammals, aquatic creatures were conveniently grouped as fishes, frogs, and water-fleas. Hermit- crabs that ascend mountains, robber-crabs that climb cocoa-nut palms, river-crabs such as are known in Eastern Europe, and that mischievously abound in Himalayan rice-fields, subtle and audacious land-crabs, like those for which the West Indies are notorious, have no representatives in England. Though our coasts and shores are rich in Brachyura, not a single species either normal or abnormal has ventured to explore and settle beyond the limits which are reached by sea-water. On the other hand, the Macrura, or long-tailed Malacostraca, are represented by a species of no mean interest, the river crayfish. In this, indeed, English- men of the present generation have reason to feel a particular pride. It was made the subject of an introduction to zoology at large by their celebrated countryman, the late Professor Huxley. Wishing to exemplify the general truths respecting the development of his favourite science by the study of a special case, he selected the common crayfish as an animal which, he says, ' taking it altogether, is better fitted for my purpose than any other.' ' It has a further historical importance. In the class of Crustacea there is scarcely any peculiarity more striking or more general than that of exuviation, the sloughing of the outer coat in its entirety. This ecdysis, or putting off of the hardened external cuticular layer, by which the growing crustacean at intervals of its life is enabled to expand its dimensions, has been often studied, but it was first thoroughly investigated by Reaumur in the case of the crayfish. Since these animals are superficially, in everything but size, un- commonly like lobsters, it is natural to ask in what the difference consists. Really the distinctions are rather numerous. The rostrum or beak of the crayfish has a single tooth on each lateral margin, that of the lobster has on each side three teeth. In both forms the tail-part or pleon has six articulated segments and a terminal plate called the telson, but this last piece is cut across by a transverse suture or quasi-articulation in the crayfish, and not so in the lobster. In the lobster all the part in front of the pleon, though representing the fourteen segments proper to the head and trunk, is in fact consolidated, but in the crayfish the last of the four- 1 The Crajfish, an Introduction to the study of Zoology, International Scientific Series, vol. xxviii. 3rd ed. p. 5 (1881). lOI
