Make out the two distinct regions or _divisions of the body_ (Fig. 81). The anterior (front) region is called the head chest or cephalothorax, and the posterior (rear) region is called the tail. Which region is larger? Why? Which is flexible? Why? Is the _covering_ of the body hard or soft? What is the advantage of such a covering? What are its disadvantages? How is the covering modified at the joints to permit motion? [Illustration: FIG. 84.—FOURTH ABDOMINAL SEGMENT OF CRAWFISH with swimmeret. ] =Tail.=—How many joints, or segments, of the tail? (Figs. 81, 83.) Does the hard covering of each segment slip under or over the segment behind it when the tail is straight? Does this lessen friction while swimming forward? Is there a pair of _swimmerets_ to each segment of the tail? (Figs. 82, 86.) Notice that each swimmeret has a main stalk (protopod), an outer branch (exopod), and an inner branch (endopod) (Fig. 84). Are the stalk and the branches each in one piece or jointed? The middle part of the tail fin is called the telson. By finding the position of the vent, decide whether the food tube goes into the telson (Fig. 82). Should it be called an abdominal segment. Are the side pieces of the tail fin attached to the telson or to the sixth segment? Do these side pieces correspond to swimmerets? Do they likewise have the Y-shaped structure? (Fig. 86.) [Illustration: FIG. 85.—1, mandible; 2,3, maxillæ; 4,5,6, maxillipeds. ] If the swimmerets on the first abdominal segment are large, the specimen is a male. If they are small, it is a female. Which sex is shown in Fig. 82? Fig. 86? [Illustration: 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.