How To Study Living Animals


Morphology of a Crayfish
Definitions.

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 herunterladen.

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 herunterladen.

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 microsoft office kostenlos downloaden vollversion deutsch chip.

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?

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? Assuming 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 antennæ; 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 class 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.