Fig. 86.—Crayfish (ventral surface).
Carapace.—The covering of the head chest (cephalothorax) is called the carapace. Has it free edges? The gills are on the sides of the body and are covered by the carapace (Fig. [87]). The projection in front is called the rostrum, meaning beak. Does the rostrum project beyond the eyes? There is a transverse groove across the carapace which may be said to divide the head from the abdomen. Where does this groove end at the sides?
Legs.—How many legs has the crayfish? How many are provided with large claws? Small claws? Is the outer claw hinged in each of the large grasping pincers? The inner claw?
Appendages for Taking Food.—If possible to watch a living crayfish eating, notice whether it places the food directly into the mouth with the large claws. Bend the large claws under and see if they will reach the mouth.
Attached just in front of the legs the crayfish has three pairs of finger-like appendages, called foot jaws (maxillipeds), with which it passes the food from the large pincers to its mouth (Figs. [85], [86]). They are in form and in use more like fingers than feet. In front of the foot jaws are two pairs of thin jaws (maxillæ) and in front of the thin jaws are a pair of stout jaws (mandibles) (Fig. [85]). Do the jaws move sidewise or up and down? Which of the jaws has a jointed finger (palp) attached to it? Do all the appendages for taking food have both exopod and endopod branches on a basal stalk or protopod? Which of the appendages have a scalloped edge? How would you know from looking at the crayfish that it is not merely a scavenger? Why are there no pincers on the hind feet?
Fig. 87.—Gill cover removed and gills exposed.
Mp, gill bailer.
Sense Organs.—Find the antennæ, or long feelers (Figs. [82], [90]). Are the antennæ attached above or below the eyes? (Fig. [87].)
