How To Study Living Animals


Definitions.

Body cavity,

the space between the body wall and the alimentary canal.

Septa (singular, septum),

the thin walls between somites, seen when the worm is opened herunterladen.

Pharynx,

the hard-walled, rather bulbous, anterior portion of the alimentary canal.

Esophagus,

the portion of the alimentary canal extending back from the pharynx with thinner walls and smaller diameter herunterladen.

Crop,

the short, wide portion of the canal back of the esophagus.

Gizzard,

the hard-walled, short region, just back of the crop microsoft office kostenlos downloaden vollversion deutsch chip.

Stomach-intestine,

the portion of the canal reaching from the gizzard to the anus.

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.

Directions.

Select a large worm and cut carefully through the body wall along one side, midway between the dorsal and ventral surfaces, from the anterior end to the posterior. Lay the worm on any convenient fairly soft surface (a piece of pine, cork, peat, paraffin), preferably under water, and pin out the walls so that you can see into the interior.

Identify the structures defined above, as well as the dorsal and ventral blood vessels and the “hearts.”

The nephridia are not easily distinguished, though they are very numerous. They are long, slender, coiled tubes, two in each somite, lying in the body cavity, one on each side of the alimentary canal. If possible, identify them.

Notice that most of the internal organs are free from the body wall, lying free in the body cavity.

Questions.

1. What is the extent of the body cavity, anteriorly and posteriorly? What is its shape?

2. What, in general, is the shape of the food canal? How many external openings has it?

3. Into what regions is the food canal differentiated? Suggest one advantage of having these specialized regions.

4. How is the alimentary canal of the worm kept away from the body walls? Why have it thus supported?

5. What is a septum? How many septa are there? What vessels and tubes pass through a septum?

6. Locate the nerve cord. How long is it? How frequently do the ganglia occur on it? Which end of the living worm is the more sensitive. Suggest the connection between this fact and the location of ganglia.

Suggested drawings.

a. Earthworm, showing structures mentioned in this study.