_Cephalo-thorax_, the anterior half of the body, divided into the head and the thorax. _Cervical groove_, the groove dividing the head from the thorax. _Abdomen_, the posterior half of the body, consisting of a number of somites. _Note._--The central part of the tail fin is usually included as a somite. _Carapace_, the continuous shell-like portion of the exoskeleton covering the cephalo-thorax. _Rostrum_, the sharp projection of the carapace at the anterior end. _Gill chamber_, a pocket on each side of the thorax, covered by a flap of the carapace. _Appendages_, paired structures attached to the body. They are named as follows:-- _Eyestalks._ (These are not classed as appendages by all students.) _Antennules_, the small feelers. _Antennæ_, the large feelers. _Mandibles_, the jaws, one on each side of the mouth. _Maxillæ_, the two pairs of small mouth parts just back of the mandibles. _Maxillipeds_, the three pairs of appendages between the maxillæ and the large claws. _Chelipeds_, the large claws or pinchers. _Walking legs_, the four pairs of appendages back of the chelipeds. _Swimmerets_, the appendages on the abdomen. _Openings_, five on the ventral surface, as follows:-- The openings from the excretory organs, through small white cones on the bases of the antennæ. The mouth, farther back, between the maxillipeds. The anal opening, in the last segment of the abdomen. The opening from the reproductive gland, toward the posterior part of the thorax. _Observations._ 1. How large is your specimen? How does it compare in size with other crayfishes in the laboratory? 2. Describe the shape of the body, contrasting the anterior end with the posterior, and the dorsal surface with the ventral. 3. Study the amount of motion permitted in different parts of the body. What prevents motion? What permits it? Where is the body most flexible? Why? Where is it most rigid? Why? 4. How much of the surface is covered with exoskeleton? What arrangement is there to permit the animal to feel contact? How can the animal grow with such an exoskeleton? 5. Place a dead crayfish in dilute acid for a few hours. What is the result? What has the acid done? Explain the fact that crayfishes are often found alive and well with a soft shell? 6. Compare the cephalo-thorax with the abdomen as to size, shape, and flexibility. 7. How many somites are there in the abdomen? Which way does it bend? Study the somite shells on every side and then state what there is in their construction which determines the direction and amount of their motion. How are the somite shells arranged to protect the body during bending? How is the ventral surface of the abdomen protected?