A Guide for the Study of Animals Part 20

_a.n.a.l opening_, the posterior opening of the food ca.n.a.l.

_Setae_ (singular form, _seta_), small bristles or stiff hairs. In the earthworm these are set in the body wall at definite intervals, and aid in locomotion.

_Cuticle_, in the earthworm a delicate, s.h.i.+ning 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 a.n.a.l 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 setae 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 gla.s.s, so that you may see through it more easily, and identify the blood vessels encircling the digestive ca.n.a.l, 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 ca.n.a.l, or alimentary ca.n.a.l, 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 ca.n.a.l? 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.

External Morphology of Earthworms

_Materials._

Preserved earthworms, the larger the better.

_Observations._

1. In what respects are the dorsal and ventral surfaces alike? In what respects different? Why?

2. Why are the right and left sides alike?

3. In what respects are the two ends alike? In what different? Why?

4. How many somites are there from the anterior end to the girdle?

How many under the girdle? How many from the girdle to the posterior end?

5. Where are the setae located in a somite? How are they distributed over the body?

_Suggested drawings._

a. An earthworm, dorsal aspect.

b. An earthworm, ventral aspect.

c. An outline diagram of a cross section, to show the location of the setae, the blood vessels and the alimentary ca.n.a.l.

Internal Morphology or Anatomy

_Materials._

(1) Preserved earthworms, as large as you can obtain. (2) Cross sections of earthworms. (3) Longitudinal sections of earthworms.

_Definitions._

_Body cavity_, the s.p.a.ce between the body wall and the alimentary ca.n.a.l.

_Septa_ (singular, _septum_), the thin walls between somites, seen when the worm is opened.

_Pharynx_, the hard-walled, rather bulbous, anterior portion of the alimentary ca.n.a.l.

_Esophagus_, the portion of the alimentary ca.n.a.l extending back from the pharynx with thinner walls and smaller diameter.

_Crop_, the short, wide portion of the ca.n.a.l back of the esophagus.

_Gizzard_, the hard-walled, short region, just back of the crop.

_Stomach-intestine_, the portion of the ca.n.a.l reaching from the gizzard to the a.n.u.s.

_Ventral nerve cord_, a light-colored thread lying against the inner surface of the ventral body wall.

_Nerve ganglia_ (singular, _ganglion_), slight swellings on the ventral nerve cord.

_Nerve ring_ or _collar_, a pair of nerves extending from the ventral nerve cord around the pharynx to a pair of ganglia (often called the "brain") in the dorsal region of the anterior end.

_Kidney tubes_ or _nephridia_, the excretory organs of the earthworm, occurring as slender, paired tubes in nearly every somite.