Place the animal on the table or desk. Notice that the body consists of an anterior, unsegmented portion, the cephalothorax (corresponding to the head plus the thorax of an insect), and a posterior, jointed abdomen. Watch the movements of the stalked eyes, the feelers and legs; allow the claws of the largest pair of legs (the “pincers”) to grasp a pencil. Put the crayfish in a white dish, with an inch or two of water, and by means of a pipette discharge a few drops of water containing some suspended colouring matter, such as carmine or indigo, near the point of attachment of the last walking leg. Describe the movements of the coloured water. When the animal is startled, notice the sudden vertical flexure of the abdomen, by means of which the body is pulled backward in the water. Feed the crayfish with meat or worms, and try to see the action of the jaws. (b) External characters.—Kill the crayfish instantaneously by [Pg 373] dropping it into boiling water. Compare the animal with a cockroach (p. 349). Examine more closely the stalked eyes, the two pairs of feelers, the four pairs of walking legs and the single pair of large pincers, and the fusion of the head and thorax to form the cephalothorax. Count the segments of the abdomen. (c) The appendages.—Examine the ventral surface of the abdomen, and notice that each segment, except the last, bears a pair of small jointed organs; these are called swimmerets. Remove one of the pair carried by the last segment but two, examine and draw it; make out that it consists of a stalk and two branches. Notice that the branches of the swimmerets of the last segment but one are expanded, and form, with the last segment, the tail fin. Study the appendages of the cephalothorax from behind forwards. They consist of (i) four pairs of legs used for walking, and a larger pair of pincers; (ii) three pairs of foot-jaws or maxillipedes; (iii) two pairs of maxillae; (iv) one pair of mandibles; (v) two pairs of feelers. (d) The gills.—With strong scissors cut off the side part of the exoskeleton of the cephalothorax, and notice the plume-like gills in the gill-chamber thus laid open. Move the adjoining legs, and see that some of the gills also move.