How To Study Living Animals


The Living Earthworm
Materials.

Living earthworms, some of which are left undisturbed from day to day, in damp earth with leaves of various plants scattered upon it.

Definitions.

Anterior end,

the head end, usually the leading end.

Posterior end,

the end opposite the anterior end herunterladen.

Ventral surface,

the lower surface, usually the one which contains the mouth.

Dorsal surface,

the one opposite the ventral surface.

Somites,

the rings or segments of which some animal bodies are composed.

Bilateral symmetry,

the symmetry usually shown by animals which have differentiated dorsal and ventral surfaces, and right and left sides herunterladen. Animals which do not have such differentiated surfaces are usually radially symmetrical, but sometimes asymmetrical (without symmetry).

Girdle,

the somewhat transparent band frequently found near the anterior end of an earthworm.

Anal opening,

the posterior opening of the food canal.

Setæ (singular form, seta),

small bristles or stiff hairs microsoft office kostenlos downloaden vollversion deutsch chip. In the earthworm these are set in the body wall at definite intervals, and aid in locomotion.

Cuticle,

in the earthworm a delicate, shining cover over the body.

Egg capsules,

small, light-colored, spindle-shaped sacks, about the size and somewhat the shape of a grain of wheat, containing the eggs or young of earthworms.

Directions.

Take a living earthworm to your table and keep it damp by placing it in a wet tray or upon moist paper. Identify the anterior and posterior ends, the dorsal and ventral surfaces, and the right and left sides. Identify also the somites and the girdle, the mouth with its projecting lip, and the anal opening.

Observations.

1. Watch a living worm for some time. Does it seem to have a definite object in its moving? If so, what is it? Upon what sense or senses does it seem to depend for guidance? Which end usually leads? Why?

2. Over what sort of surface does it move most easily? Why? Watch it closely for some time and discover how it is able to move from place to place. (Suggestion. What is the function of the setæ in this process? How can you explain the alternate contraction and expansion of parts?)

3. From time to time, for perhaps a week, examine the leaves which were scattered where the worms could reach them. Have the worms moved them about at all? If so, where are the leaves left? Have any been eaten, in part or entirely? If so, is there any evidence of selection, either as to the kind of leaf or the portion of leaf eaten? If earthworms select food, what senses would be useful for the purpose? Have you any evidence that earthworms possess such senses?

4. Looking through the dorsal wall, notice the meandering red line, seen more easily in some regions than in others. This is the dorsal blood vessel. How long is it? Where is it wider? Where narrower? Notice its pulsations. How many times per minute does it pulsate? In which direction is the blood forced? Is there a corresponding ventral blood vessel? Place a small worm between two pieces of glass, so that you may see through it more easily, and identify the blood vessels encircling the digestive canal, near the anterior end. These are the so-called “hearts” of the earthworm. If possible, decide in which direction the blood flows through them.

5. The food canal, or alimentary canal, lies underneath the dorsal blood vessel, and is usually easily seen, especially if it is full of food. Notice it when the worm is fully stretched and again when it is contracted. How long is the canal? Why does it wrinkle when the worm contracts? Where does it open to the outside? Why does it need to?

6. Where do you infer respiration must take place in this animal? Why do you think so? What fits this surface for such a purpose? Why does an earthworm seem so uncomfortable when it is too dry?

7. Where do earthworms live? What conditions are necessary in their habitat?

8. When do earthworms usually leave their burrows? Why at that particular time rather than at another? Why does “the early bird catch the worm”?

9. What enemies do earthworms have? How are they protected against these enemies?

10. If you have found egg capsules when collecting worms, describe them.