c. An outline diagram of a cross section, to show the location of the

setæ, the blood vessels and the alimentary canal.

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 space between the body wall and the alimentary canal.

_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 canal.

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

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

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

_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.

Details of Structure--Microscopic Anatomy

_Materials._

Sections of earthworms, preferably both cross sections and dorso-ventral, longitudinal ones.

_Directions._

In a section under a simple lens, identify the dorsal and ventral surfaces, the body wall, the body cavity, the alimentary canal, and, if possible, the dorsal and ventral blood vessels and the ventral nerve cord.

Under a microscope identify the same structures. Notice that the body wall consists of three layers of cells: an outer single layer, the _epidermis_; a middle layer, the _circular muscles_; and an inner one, the _longitudinal muscles_.

The nephridia show as loosely scattered fragments in the body cavity, at the right and left of the alimentary canal.

If you happen to have a section which shows one or more setæ, identify the muscles which operate it, and the group of glandular cells at its inner end, which are known as _setigerous_ (from _seta_) cells.

_Questions._

1. Describe the epidermal cells. What is their probable function? Among them notice larger cells, clear and rounded. These are the _mucous_ (slime) cells.

What is the use of mucus to the worm?

2. Describe the muscle cells. In which direction do the muscle fibers extend? What is their function? Which layer of muscle cells is thicker, the circular or the longitudinal? Why should it be?

3. Notice the cells in the walls of the alimentary canal. What layers do you find? How are they arranged?

4. If the section you are studying is a cross section from the region back of the gizzard, the alimentary canal will look horseshoe shaped, indented from the dorsal surface. What is the effect of this indentation upon the amount of surface in the alimentary canal?

5. Study the cells of the nerve cord. How do they compare in size and shape with the muscle cells?

_Suggested drawings._

a. A diagram of a cross section, showing the relation of the organs.

b. A diagram of a longitudinal section, at least through the body wall, to show the arrangement of muscle fibers.

c. A drawing of a portion of the body wall, to show details.

Summary of Important Points in Study of the Earthworm

1. Compared with a hydra, how many cells has an earthworm?

2. Compared with a hydra, how much are the cells of an earthworm differentiated?

3. How are these differentiated cells usually arranged with respect to one another? What advantage is there in this arrangement?

4. Recall the kinds of work done by paramecium, sponge, hydra, and worm, and at the same time consider also the efficiency of each. Can earthworms do any more kinds of work than any of the others? Can they do any more work? Can they do any of it better? Give the probable reasons for this?

Comparative Study of Worms

_Materials._

As many different kinds of worms as you can get, living or dead.

_Directions._

Identify your specimens. Then study as many as your time will allow, using these general questions for each:--

_Questions._

1. How large is the specimen and what is its shape?

2. Can you distinguish a head or a head end? If so, by what peculiarities?

3. State whether the body is segmented or not, and, if it is, whether the segments are alike in form and appearance, _i.e._ whether the segments are uniform.

4. State whether the animal is bilaterally symmetrical, radially symmetrical, or without symmetry.

5. Compare this worm with the earthworm as to sense organs.

6. What organs for respiration has it?

7. What special protective devices has it?

8. If possible, find out and state where this worm lives. What can you see in the structure of this worm which enables it to live where it does?

Summary of the Comparative Study of Worms

1. Name the different worms you have studied. What characteristics have they in common?

2. What different methods of obtaining food do they show?

3. What variations do they show in senses? in sense organs?

4. Which one seems to you best adapted to its habitat? In what ways?

_Suggested drawings._

a. One drawing of each worm studied.

Review and Library Work on Worms

1. What are the distinguishing characteristics of worms?

2. Give the classes of worms, and the authority for this classification.

3. What kind of soil do earthworms seem to prefer? Why should they? How do they form their burrows? What are the castings around the mouth of a burrow? How are they placed there?

4. In what ways do earthworms benefit the soil? How great is their effect estimated to be?

5. Give a brief sketch of the life of Charles Darwin, noting especially the work he did with earthworms.

Why is Darwin's work on earthworms noteworthy: because it is such a large proportion of the work he did, or because it is so much of the work which has been done on earthworms?

6. How are earthworms protected against the cold of our winters? What limits the northern range of earthworms?

7. Where are earthworms found geographically? Why are they so widely distributed? By what means are they extended from one locality to another?

8. How do earthworms reproduce? What care do they take of their young?

9. What tissues or organs of earthworms correspond in function with the ectoderm of hydra; with the endoderm? Why does an earthworm need a system of blood circulation more than a hydra does?

10. Contrast the number of openings in an earthworm's alimentary canal with the number in a hydra's digestive cavity. Which plan seems a better one? In what respects?

11. Contrast a cross section of hydra with one of earthworm as to the number of cavities. Which seems to you the better plan? Why?

12. Why does a nereis need more respiratory surface than an earthworm does?

13. Comparing earthworm and nereis, in what respects is the earthworm degenerate? How does it manage to succeed so well with such a degenerate body?

14. What is a parasite? How many hosts does a typical parasite require for its development? Which host is known as the intermediate one?

15. Trace the history of a tapeworm from the egg to the adult. At what stage are they most likely to be destroyed? What provision is there for this? What advantages are there to the host in the fact that a tapeworm's egg cannot develop in the original host? What advantages to the parasite?

16. What organs has a parasite lost, if it ever had them? How does it succeed without them? What connection is there between parasitism and degeneration? Can you decide which is cause and which is effect? If so, which is?

17. Why do worms so easily become parasitic? What advantages are there in becoming a parasite? What disadvantages?

18. What is radial symmetry? Name two animals which show it. What is bilateral symmetry? Name two animals which show it. What is the relation between locomotion and symmetry?

19. What is meant in biology by the term "regeneration?" To what extent have we this power? To what extent have hydra and earthworm? What are the results of this power?

20. Name various methods of locomotion among worms. Give examples. Name a fixed or sedentary worm.

21. What is the economic importance of worms? Consider here not only earthworms and tapeworms, but also the stomach worms of sheep, liver flukes, trichinæ, hookworms, vinegar eels, and as many others as you have time and books to look up.

5. THE CONNECTION BETWEEN STRUCTURE AND FUNCTION

_A Review of the Work done on the First Four Groups of Animals_

Review all your studies on the protozoa, sponges, coelenterates, and worms. Write the results in the following summary:--

1. What work, _i.e._ labor, must an animal do to live?

2. How many cells are necessary to do this work?

3. When this work is divided among a number of cells, what is the effect upon the quantity and quality of work accomplished?

4. When this work is divided among a number of cells, how does the structure of the cells show it? How does the arrangement of the cells also show it? Give examples.

5. The technical expression for this specialization of cells, giving them different functions, is "division of labor." Formulate a clear definition for this expression, giving an example to illustrate it.

6. Is division of labor a good thing for an animal body, or is it not? Give reasons for your opinion, with examples for illustration.