_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 antennae.

The mouth, farther back, between the maxillipeds.

The a.n.a.l 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 sh.e.l.l?

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 sh.e.l.ls 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 sh.e.l.ls arranged to protect the body during bending? How is the ventral surface of the abdomen protected?

8. Where are the appendages attached? Study a walking leg and describe its general construction, the number and kind of joints, the direction of motion in each joint, and the range of motion for the whole leg. Study an antenna in the same way. What methods are used in the crayfish to secure a wide range of motion? To secure flexibility?

9. Carefully split a crayfish into right and left halves. To do this, first cut through the ventral exoskeleton from end to end with scissors, then with a sharp knife or razor cut through to the dorsal exoskeleton and cut that with the scissors. Study one half, to get a better idea of the attachment of the appendages. These may then be removed and placed in order on a piece of paper upon which a list of the appendages has been written.

10. How many pairs of appendages are there? How may they be grouped according to location; how grouped according to function? How many pairs are there in each group?

11. What similarities of structure do you find in nearly all of the appendages? a.s.suming a swimmeret of the third, fourth, or fifth somite to be the least changed from the primitive type, what changes were necessary to make the sixth swimmeret; the third maxilliped; the walking legs; the antennae; the antennules?

12. Remove the part of the carapace which covers a gill chamber. What are the boundaries of the chamber? Where does it open to the water?

13. Describe the appearance and the texture of a gill. How are the gills kept moist when the crayfish is in water; when it is on land?

Why should they be kept moist?

14. Would you cla.s.s the gills as external structures or as internal?

Why do you think so? To what are they attached? How are the gills affected by the motion of the legs?

15. What work goes on in the gills? How is the supply of oxygen renewed? In this connection, try a live crayfish, kept quiet in water just about deep enough to cover it. Float bits of paper near it or carefully place a drop of ink in the water near it. By some such method currents of water may usually be shown, and their direction determined. Consider also the habitual motions of mouth parts and swimmerets, the bubbles sometimes seen when a crayfish is dropped into water and the habit crayfishes have of lying on one side, close to the surface of the water.

Summary of the Study of Crayfishes

To summarize your study, write a connected account of the relations of crayfishes to their environments, under the following topics:--

1. What are the varying conditions in their surroundings which crayfishes must meet? Which are most important?

2. What conditions must be maintained in order that crayfishes may succeed, _i.e._ may live and reproduce?

3. How does a crayfish know what are the conditions around it?

4. How is it fitted to meet these conditions? (Answer in the following details):--

a. How wide a food range has it, _i.e._ how many kinds of food does it eat? How does it find its food? How does it reach it? How does it take its food? How does it make food small enough to be eaten?

b. What are the organs for taking in oxygen? Where are they? How are they attached? How is the supply of oxygen kept up? How are the organs kept from drying, from clogging, and from mechanical injury?

c. What ranges of temperature can crayfishes endure? What temperature is best? How do they avoid fatal extremes?

d. What are the enemies of crayfishes? What protection against these have they?

e. How often do crayfishes reproduce? About how many times during a normal lifetime? About how many eggs are there and how many of them hatch? What care is given to the eggs and to the young? About how many of the young reach maturity? (_Suggestion._ Do the crayfishes of a region vary noticeably in numbers from year to year?)

5. What limits the range of crayfishes, north and south? What limits it on land? What in water?

6. When the crayfishes of a given locality are not well adapted to it, what can they do?

_Suggested drawings._

a. The whole animal, dorsal surface, preferably without appendages.

b. One of each pair of appendages, except where they duplicate.

c. The tail-fin. Label the sixth swimmerets, the sixth and seventh somites.

d. The gill chamber, with gills in position. Show circulation of water by arrows.

e. A gill, to show construction.

2. COMPARATIVE STUDY OF CRUSTACEA

_Materials._

Get together all the different specimens of crustacea you can collect, and identify the material. Then study each specimen as follows:--