A HISTORY OF HERTFORDSHIRE for the marine lobster, and the specific name jluviatilis for a kindred species of crayfish found in various parts of the European continent, but not in England. Although the common lobster and the common shrimp are easy to obtain inland, as a scientific object of investigation the crayfish has an advantage over both by hitting the happy mean between over large and over small. But all three are convenient to handle, and may be used together to throw light upon the fascinations of comparative anatomy. On the other hand, for the observing of manners and customs, of the arts and crafts, the dwelling-places and the breeding-habits of the living creature in a word, for all that concerns the now popular study of biology the river crayfish is of unique importance to Midland carcino- logists. In a general way almost every one is willing to admit, with or without reserve, the philosophical axiom that nature makes nothing in vain. Yet scarcely to any one, except a thoughtful expert, will it readily occur to suppose that there can be any special philosophy in the joints of the leg or the body-segments of a Potamobius, a creature made to be eaten, an unconsidered trifle, the garnish of a dish ! It is not that there has been any such want of appreciation on the part of naturalists, for they at intervals for centuries past have studied this genus with almost loving care. Already in the middle of the sixteenth century observation and experiment were brought to bear upon it. Like man himself, it is tolerably omnivorous. Like so many other crustaceans, it is in part a scavenger. Vegetable food is welcome, but perhaps animal food and offal even more so. Thus the old author Gesner states that if the car- case of a horse or dog or any other animal be submerged, the crayfishes presently like vultures gather about it in swarms, not to quit till every morsel of flesh has been eaten off. He tells also of a man who could not help thinking that these swarms must be generated from the horse's body, like the bees of Aristasus from the corrupting entrails of a slaughtered bull. But when this person had from time to time thrown dead horses into the water, the result of his experiments weaned him from his poetical fancy. 1 The several illustrious men who between Gesner's time and Huxley's have studied the crayfish in various aspects might be thought to have exhausted the subject. But the comparatively recent work of Dr. Theodor List on the motor apparatus of the Arthropoda shows that this is by no means the case. Dr. List tracks the crayfish to its favourite brooks, observes its preference for proximity to a bridge, where it may find places of ambush and shelter from the odious daylight. He descries it lurking among the stones in the bed of the rivulet, with only its large claws emergent, in readiness to snap the passing prey. ' If you attempt to catch it, you become aware that the abdomen, which it flexes several times in rapid succession, is a capital locomotive apparatus. Otherwise by day it is a very lazy customer. But as soon as darkness has set in it leaves its hiding-place and goes on the prowl. With the great claws stretched in advance, the large antennae feeling about in all 1 Gesner, Hiitorue Animalium, liber iv., 1558 (ed. 1604, p. 105). 182
