The science of life : fully illustrated in tone and line and including many diagrams

THE SECOND GREAT PHYLUM: THE ARTHROPODS

the Copepods, exceeding in multitude any other kind of marine animal except the unicellular ones, and providing one of the most important sources of food of fishes. The late Sir Arthur Shipley, in his delightful Hunting Under the Microscope, says: “Ifona sea voyage you tie a piece of bolting cloth loosely to the bath-tap which introduces thesalt water into the bath, in avery short time you will find a deposit or paste at the bottom, which in the main consists of copepods.”’ The whole of the sea is like that. 1 There are more copepods in Plymouth Sound than there are people in the world—indeed, there are hundreds of times more.

We may note that crustaceans, like the great majority of arthropods, enclosed as they are by a hard, unyielding coat of armour, have to moult that coating off from time to time so that they may grow. The life of a lobster, for example, is a rhythmic alternation of two states. For a time he liyes in his shell, not growing, but storing in his liver the materials necessary for a short but energetic burst of growth; then he retires to a safe crevice in the rock and casts off his armour—not only the outer armour, but even the lining and teeth of his stomach and the lining of his intestine—and, temporarily defenceless and helpless because of the softness of his appendages, he uses the stored material to grow and then to build a new and more roomy skeleton. This done, he emerges from his retreat, bigger and stronger for his reconstruction, into a warring world.

Fig. 82. The parasite Sacculina growing (at X) under the tail of a crab and sending hungry roots into all its organs.

The Crustacean affinities of this creature are evident in its lwo young stages, seen, highly magnified, at 1 and 2.

During the earlier stages of their lifehistories the crustacea generally undergo one or more profound changes of bodily shape. The young are very unlike the old. We need not catalogue the different larval forms that are found in the various groups ; some of them are shown in the illustrations herewith.

The most striking changes of shape during the life-history are seen in the group to which the barnacles belong. ‘There is no very evident resemblance between the lobster, with which we began our account of the crustacea, and an adult barnacle, permanently fixed as it is to a rock or to some other hard object, shut up in a little box of skeletal plates, and feeding on minute organisms in the sea-water which it entangles by means of lashing movements of a cluster of long, feathery, whip-like arms. Never-

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