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

BOOK 2

And now for a rapid review of the five great classes of arthropods, to note the main changes that have been rung on this underlying theme.

§ 3a

Shrimps, Crabs, Water-fleas, and Barnacles

The first class of arthropods, the Crustacea, includes a great and varied assemblage of forms. Nearly all of these animals are aquatic, breathing by means of gills, and most of them are marine—indeed, they are the only great group of marine arthropods. The more familiar forms—familiar because they are the largest and also because they are edible—are the lobsters, crayfish, prawns, shrimps, and crabs. These animals are all very similar to each other. A crab is essentially a lobster who has given up the abdomen-flapping method of swimming, and whose abdomen, small and feeble, is permanently bent forward under his broadened cephalothorax. The hermitcrabs also belong to this group; they are lobster-like creatures whose abdomen is not hard, but soft and delicate, and who have therefore to wear the deserted shell of a dead sea-snail on their hinder ‘parts, not for

THE SCIENCE OF LIFE

Fig. 81. The growing Hermit Crab changes into a roomier shell (1—4).

CHAPTER 2

modesty, but as a protection against the many prowling and hungry creatures of the sea.

Other crustaceans, not quite so well known, are the wood-lice and the group including the so-called fresh-water shrimps, which differ in a number of important points from the true shrimps.

Besides these easily visible forms there are several groups of minute crustaceans, seen only as whitish specks to the naked eye, but nevertheless of great economic importance. Cyclops, a one-eyed, literally heartless crustacean, hardly a millimetre in length, swarms in countless numbers in lakes or pools or slow streams ; this and the slightly larger Daphnia are among the commonest inhabitants of fresh water. The name “ water-fleas,”’ often applied to these animals, is misleading, because they feed on even more minute organisms and not, as fleas do, on the blood of larger creatures. Daphnia is particularly and deservedly popular among amateur microscopists because her body (nearly all Daphnias are female) is so transparent that all her living organs, her beating heart and her churning bowel, can be watched with ease under a low power. In the surface layers of the open sea there are prodigious numbers of minute crustaceans belonging to the same group as Cyclops,

=a ee as os se] Below on the right is

another crab with sea-anemones on his shell—a commonly found partnership between two very different kinds of creature.

124.