The crab (Fig. 231) is markedly different in shape from the crayfish or lobster, but is nevertheless easily seen to be built on essentially the same lines of structure. It also consists of twenty segments, of which the first thirteen are fused to form a [Pg 377] cephalothorax; and the appendages of this region are quite comparable with those of the crayfish. The great width of the shell is largely due to the gill-covers, which stand out from the sides of the body much further than do those of the crayfish. As a result the crab finds it easy to run side-first. The crab is essentially a walking, not a swimming, animal, and the abdomen—upon which the crayfish and lobster so much depend in swimming—is in the crab reduced in size and kept tucked under the cephalothorax. Fig. 232.—The Wood-louse. (× 1.) Crustaceans.—These facts are sufficient to show that the title “insect” cannot with any propriety be given to either the crayfish or the crab, unless, indeed, the term is to be applied to all arthropods indiscriminately. The crayfish, lobsters, shrimps, prawns, crabs, barnacles, and many less-familiar animals, are placed in the crustacean class of arthropods. Crustaceans have usually a distinct head, thorax, and abdomen; but some of the thoracic segments may be fused with the head to form a cephalothorax (p. 375). Like other arthropods, the animals are covered with an armour of chitin, and in many cases this is so hardened, except at the joints, by mineral matter that it becomes a rigid shell or crust. The head bears two pairs of feelers in addition to the jaws; and the segments of the thorax and abdomen are provided with appendages which are variously modified as jaw-feet, legs, swimmerets, etc. Typically, the animals breathe by gills and are aquatic, but forms are known which are able to live comfortably on land if the gills are kept moist. One of the most interesting examples of this is found in the common wood-louse (Fig. 232), which lurks under stones and logs in damp and dark situations, and breathes by plate-like gills on the abdominal segments. [Pg 378] Other arthropods.—Other familiar arthropods, which are neither insects nor crustaceans, are spiders and centipedes. The spiders belong to the arachnid class, and the centipedes to the myriapod class of arthropods. 66. SOME COMMON MOLLUSCS.[37] 1. The fresh-water mussel.—(a) Habits.—Study the habits of the living animal (Fig. 233) in the aquarium. Notice the muscular foot which is protruded between the halves (valves) of the shell, and by means of which the mussel slowly makes its way along the sandy bottom. With a pipette, discharge some water, coloured with carmine or indigo, close to the more pointed end of the shell. Where is the coloured water taken in, and where is it expelled? Notice that the shell closes when the animal is handled.