CHAPTER III. The Connection Between Structure and Function

THE CONNECTION BETWEEN STRUCTURE AND FUNCTION

1. A STUDY OF THE CELL AND OF PROTOZOA

To show what Single Cells can Do

Materials.

Some single cells of plant or animal tissue, stained to show structure. Slides of a one-celled animal, stained. Living one-celled animals.

Definitions.

Cell, the smallest living unit.

Protoplasm, the living material composing the cell.

Nucleus, a dense bit of protoplasm, usually near the center of the cell, often staining dark.

Cytoplasm, the less dense protoplasm outside of the nucleus, usually taking a lighter stain.

Nucleolus, paranucleus or micronucleus, a very small, dense, dark-staining body, either within the nucleus (nucleolus) or near it (paranucleus or micronucleus)

Cell wall, the lifeless membrane surrounding many cells, secreted by the protoplasm.

Food balls, bits of food inside the cells of many one-celled animals, usually showing through the walls.

Food vacuole, a small drop of water containing digestive material and a food ball.

Contracting or pulsating vacuoles, small, clear spots in the cell, filled with water. In the living cell these disappear at intervals and then appear again.

Oral groove, a funnel-shaped groove in one side of some one-celled animals, conducting food to the mouth. In paramecium it often shows as an oblique line when the animal rolls.

Gullet, the inner end of the oral groove.

Cilia, numerous minute, vibrating, protoplasmic hairs on the surface of many cells.

Respiration, the passage of oxygen into the tissues of a living organism and of carbon dioxide out of them. These gases can pass through any thin, moist, organic membrane. When such a membrane separates two fluids which differ in the amount of oxygen they contain, oxygen passes to the fluid containing the smaller amount. The same is true of carbon dioxide. Respiration is believed to occur in all living organisms.

This passage of fluids through membranes is known as osmosis.

Digestion, the process of making food materials soluble, so that they can pass through membranes and be used to build up protoplasm. A few forms of cells are able to take in solid food and digest it in their protoplasm, but most cells can admit only fluid food.

Fission, a method of reproduction used in all cells, by which a cell divides itself into two, usually through the center. In some one-celled animals this may be preceded by conjugation, when two animals unite temporarily and exchange nuclear substance; or in some forms two cells may fuse and the resulting cell may divide. Budding is a form of fission in which a small projection is formed on the parent cell and then cut off, making a new individual.

Protozoa (first animals), animals of one cell, existing alone or in loose colonies.

Observations.

1. Examine a single cell, stained to show structure. Identify the nucleus, cytoplasm, and, if present, the nucleolus or the micronucleus, and the cell wall. Draw to show the form of the cell and the details of its structure. Label all details.

2. Examine some stained paramecia. Select a typical one and identify in it nucleus, micronucleus, cytoplasm, and cell wall or cell membrane. You may also be able to see vacuoles, looking like holes in the stained protoplasm. Give reasons for considering this animal to be a single cell. Draw one, to show its cellular structure. Label all details.

3. Clean a slide and cover glass, place a drop of water containing living paramecia on the slide, cover it, and examine. What structures do you see which you saw in the stained paramecia? What structures do not show? Identify any new structures you may observe. Identify also the leading end and the side containing the oral groove.

4. Describe the shape of the animal.

What is the actual length of the animal?

5. After watching the animal for some time, describe the path followed by a given specimen as it crosses the field of the microscope. What reason can you see, if any, why this paramecium is moving? What external factors, if any, seem to determine the path it follows?

6. How rapidly do paramecia really move? What structures do they use in locomotion?

How do they manage to move in one direction, instead of alternately backward and forward? How do they manage to move in a straight line, though their bodies are not symmetrical?

7. What is the food of the paramecia? How do they find it? Find a specimen at rest and watch the oral groove. Suggest a method by which food may be collected into it. If possible, note the process of swallowing, and the resulting food ball.

Note.--If powdered carmine be placed in the water with some paramecia, it can be seen in the food balls a half hour or so later.

8. Where are the food balls located? Watch them in an individual until you notice their motion. Where are the larger food balls? the smaller ones? Assuming them to have been of approximately equal sizes when they were taken in, how can you account for differences now?

9. Where are the contracting vacuoles? How many are there? How often does one contract?

What is their function?

10. As you have been studying paramecia, to what external influences (as contact, heat, light, etc.) have you seen them respond? How do they show it when they do respond? Is such a response an advantage to them or not? What would be the result if they were not able to detect changes in their surroundings?

11. Where does respiration occur in paramecia? Where do they obtain their supply of oxygen?

12. Among the paramecia you are studying you usually find at least one in the process of fission. Watch it until the halves separate, if you can. Compare the halves. Do they rank as parent and offspring? If so, which is which? If not, which are they, parent or offspring?

13. If you happen to find a pair conjugating, notice the process, as far as you can, in the living animals.

Suggested drawings.

a. A drawing to show all the details seen in the living paramecium.

b. A diagram to show the path followed by a paramecium to get around some obstacle.

c. Drawings to show that paramecia are constant in shape and yet flexible.

d. A drawing to show at least one stage in fission. This may be from a permanent preparation.

e. A drawing to show paramecia conjugating. This also may be from a permanent preparation.

f. Instead of all these separate drawings they may be combined into one. Represent the field of the microscope, and in it draw all necessary figures, to show the facts called for in the first five drawings and any other facts you have observed about living protozoa. Make the whole drawing to scale.

Summary of Important Points in the Study of Paramecia

1. Look back over your study of paramecia and list the different kinds of work you saw paramecia doing; also the kinds of work you infer they can do. What organs have they to use? When there is no organ to do a given thing, e.g. to digest food, how is the work done?

2. What conditions are favorable to paramecia? Why are they so numerous under favorable conditions?

3. What would you call a successful animal? Are paramecia successful? Give reasons why they are or are not.

Comparative Study of Protozoa

To enlarge your idea of what a cell can do, spend as much more time on the one-celled animals as your course will permit. Any stagnant water may furnish several kinds. By means of reference books, identify as many as you can. In each case notice:--

1. Its size, shape and general appearance, comparing and contrasting it with paramecium.

2. Its usual surroundings, i.e. the conditions it has to meet.

3. The means it has of finding out facts about its surroundings.

4. The means it has of adjusting itself to its surroundings. For example, is it stationary? If so, what does it do when conditions change? Is it locomotory? If so, how effective is its locomotion?

5. What is its food? How does it find food?

6. Can it do as many kinds of work as paramecium can? Can it do any that paramecium cannot do? If so, what?

Review and Library Questions on Protozoa

1. What are the characteristics which distinguish protozoa from other animals?

2. What are the classes of protozoa? Characteristics of each class?

3. What is digestion? Where does it take place in the protozoa?

4. What results from the fact that the amoeba has no cell wall? (Give at least two points.)

5. In what ways are paramecia more specialized than amoeba are? How does their greater specialization show in their work?

6. What different methods of locomotion are shown among protozoa? By what means is locomotion accomplished in each case?

7. What is encysting? Name some protozoa which encyst. How long may an encysted animal live? When do they encyst? Why?

8. Give methods of reproduction among protozoa. Which method is fitted for rapid multiplication, for withstanding drouth; for renewing vitality?

9. Many scientists speak of protozoa as immortal. What argument is there to support such a statement?

10. Why are no protozoa large animals? Give at least two reasons.

11. Why are protozoa so numerous? Why more numerous in stagnant water?

12. Where are protozoa found?

13. Why are protozoa so widely distributed?

14. Write the probable history of a piece of chalk.

15. What connection is there between protozoa and some polishing powders?

16. Where in the human body are malarial protozoa found? How are they transferred from one human being to another? Why is there likely to be more malaria in newly settled regions than in older ones? If you were obliged to spend some time in a region where malaria existed, what precautions would you take?

17. Name other diseases caused by protozoa. How are they fought?

18. What beneficial effect have some protozoa upon the water of stagnant ponds and ditches? How may some forms injure water for household purposes?

19. Give at least three reasons for thinking that protozoa are the most ancient animals.

20. Why are protozoa of great importance to the world?

2. A STUDY OF SPONGES

To show how cells loosely associated may work together.

Materials.

The simplest of the many-celled animals are the sponges, which, with one exception, are salt-water forms. That one, the spongilla, is not easily found and is very difficult to maintain in the laboratory. For these reasons the material for this study is very meager, except at the seashore, and much of the work must be done from diagrams and reference books. Small simple preserved sponges and complex toilet sponge skeletons will also be used.

Definitions.

Body wall, the outer wall in bodies of the many-celled animals.

Central cavity, the cavity surrounded by the body wall in the simpler many-celled animals, as in the sponges.

Canals, channels through the body walls of sponges.

Inhalent pores, the outer ends of the canals.

Ostia, the inner ends of the canals.

Osculum, the large opening of the central cavity, at the distal end of the sponge.

Spicules, tiny needles of mineral substance found in the walls of many sponges.

Fibers, flexible threads of horny material found in the walls of many sponges.

Endoderm cells, cells lining the canals. They have cilia or flagella (projections larger than cilia).

Ectoderm cells, cells covering the outside of sponges and some other animals. In sponges it is believed that endoderm and ectoderm cells are able to exchange positions and functions.

Mesoglea, a jelly-like layer between the endoderm and ectoderm layers. In the sponges this contains many wandering cells, probably from the other layers.

Porifera (pore bearers), animals with many more or less independent cells, supported by solid skeletal parts and penetrated by a system of canals which open on the surface as pores.

Directions.

Study a simple sponge to see the shape, size, and point of attachment. Identify the osculum. In a diagram of a long section of a simple sponge identify the central cavity, body walls, canals, inhalent pores, ostia, and osculum. In a simple sponge cut like the diagram identify the same structures. Do the same for the toilet sponge.

Study a diagram of a portion of the body wall, considerably enlarged. Identify the endoderm and ectoderm cells, the spicules or fibers, and, among the spicules or fibers, irregular amoeboid cells, sometimes called mesoderm cells.

Examine a fragment or section of each kind of sponge under the microscope. Notice the arrangement, shape, and length of the spicules and of the fibers.

Test both kinds of sponges by dropping a bit of each into weak acid, and noting the results. Also burn a bit of each and notice the odor.

Questions.

1. What is the shape of a simple sponge? What enables a mass of cells to retain such a definite shape?

2. What seems to be the composition of the skeletons? Why is one type of skeleton rigid and the other elastic?

3. Since sponges are attached for most of their lives to stationary objects, suggest means for obtaining food and oxygen, and for getting rid of waste matter.

4. Although individual cells are sensitive, a sponge as a whole is not. What connection has this fact with the fact that sponges are stationary?

5. Compare simple and complex sponges.

Suggested drawings.

a. A view of a simple sponge. Label everything shown.

b. A diagram of a simple sponge split in halves. Show by arrows the path followed by the water as it passes through the sponge.

c. A few spicules.

d. A few fibers.

Summary of Important Points in the Study of Sponges

1. What are two functions of the spicules or fibers?

2. What are at least two of the functions of the endoderm cells?

3. What can you suggest as functions for the ectoderm cells?

4. In what cases do cells show "team work" in accomplishing an object?

5. What degree of specialization is indicated by the fact that the cells may exchange positions and functions?

6. What work can any single cell of a sponge do? Compare the work done by such a cell with that done by a paramecium.

7. What work can a whole sponge do? Compare that with the work done by a paramecium.

Review and Library Exercise on Sponges

1. What are the distinguishing characteristics of Porifera?

2. Sponges were once supposed to be plants. In what respect are they plant-like? What made students finally class them as animals?

3. How do sponges reproduce? How are they distributed to new locations?

4. Where, as to depth of water, do most sponges grow? Where, as to oceans? Where, as to latitude?

5. What are some of the difficulties which confront a stationary animal? How are they overcome?

6. To what class of sponges do the "toilet" sponges belong? Why?

7. What conditions are necessary for toilet sponges to thrive? Where are the best ones found? Where are they most numerous? How are they collected? How are they prepared for market?

8. What is man able to do toward raising good sponges for market?

9. Using reference books and museum specimens, describe some especially odd sponges.

3. A STUDY OF COELENTERATES

To show cells working together more definitely than in Sponges

A Study of Hydra

Materials.

Living hydras in permanent aquaria, undisturbed. Living hydras in small aquaria, i.e. tumblers, test tubes, watch glasses, etc., with pieces of water weed and if possible some of the microscopic animals found in water where hydras are abundant. If kept cool, hydras may live several days in such aquaria. Permanent slides of hydras; some whole, some in sections, and some showing the organs of reproduction.

Definitions.

Proximal end, the end by which an animal is attached to an object.

Distal end, the end opposite the proximal end.

Tentacles, slender projections around the distal end.

Mouth, the opening through the distal end, into the central cavity.

Bud, a small hydra or other coelenterate growing out from the wall of the parent.

Mesoglea, a thin, gluey partition, without wandering cells, between the ectoderm and the endoderm.

Nettle cells, very small cells, chiefly in the tentacles, easily identified in permanent preparations as clear cells with small hairs projecting from them. See text-books for details of their structure.

Spermary, the region or organ where the sperm cells are formed.

Ovary, the region or organ where the egg cells are formed.

Coelenterates (hollow bowels), sac-shaped animals, the digestive tract having only one opening; the body wall is of two layers.

Directions.

Take a small aquarium to your table, set it down carefully and leave it undisturbed. Identify a hydra and watch it for some time.

Observations on the living animals.

1. Describe the size and shape of a hydra when expanded. Disturb it slightly by shaking the aquarium a little, and describe its shape when contracted. Notice also the flexibility of the body. What do you infer concerning the hydra's possession of a skeleton? What advantage can it be to have a body so flexible?

2. How many tentacles has the hydra that you are studying? What does the hydra do with these tentacles when it is expanded? What is the probable object of such actions?

3. How does a hydra respond to contact? What seems to be the object of such a response?

4. Notice the location of the hydras in the large, undisturbed aquaria. Where are they placed as regards the light side of the aquarium? Of what value is such a response to light in their case?

5. How can a hydra locate the small animals which are its food?

How can it capture them?

6. What motions may a hydra perform, while remaining attached by its base? What are the results of these movements?

7. If you have happened to see a hydra move from one place to another, describe the process. If not, give the facts which lead you to believe that it is able to do so. Suggest all the methods you think it may be able to use. What is your opinion of the hydra's power of locomotion? Of what use is it in getting food; in escaping enemies; in following the fluctuations of the water supply? If you had to class the hydra as either one, would you call it a stationary or a locomotory animal?

8. Study budding hydras. Compare the bud with the parent hydra as to size, form and number and size of tentacles. Notice whether the bud moves independently or only with the parent. When does it separate from the parent?

9. In hydras collected late in the fall you may see another method of reproduction. If such material is at hand, notice small swellings near the proximal end and others near the tentacles. Eggs are produced in the lower one, the ovary, and sperm cells in the upper one, the spermary. Refer to your text-book for further details.

Details of structure.

1. Using an entire mounted specimen and a section of hydra, identify the body wall and the central cavity. What is the extent of the central cavity? (Examine both the body and the tentacles.) Where does it open to the outside? What do you think is its use?

2. In the body wall, identify the endodermal and ectodermal layers of cells, separated by the mesoglea, which is usually stained more deeply. Study these cell layers carefully. What work ought each to do? What can you discover in its structure which would fit each layer to do its work?

3. In the tentacles, identify the nettle cells. Where are they? How are they arranged? About how many of them would be discharged if a small animal were to bump into a tentacle?

Summary of Important Points in the Study of Hydra

1. Name the different kinds of cells in a hydra. Which kind differs most from such a cell as the starfish egg? What work does this specialized cell do?

2. How much of a hydra's body may be set in action by touching a tentacle? Contrast this with the sponge. What do you infer concerning the nervous power of these two animals?

3. Look back over your notes and list the different kinds of work a hydra can do.

4. Can it do any more kinds of work than a paramecium or a sponge can? If so, give further details.

5. Can it do any of its work in any better way? Would you expect it to be able to? Why, or why not?

Suggested drawings.

a. Hydra undisturbed, and hydra after being touched or shaken.

b. A hydra in successive poses to show its flexibility.

c. A hydra taking food.

d. Hydras to show reproduction in one or both ways.

e. A section of hydra, showing details.

Comparative Study of Coelenterates

Materials.

Various coelenterates, such as hydroids, hydro-medusæ, jellyfishes, sea anemones, corals, sea fans, etc. Since nearly all the coelenterates except hydras are marine forms, these will usually have to be dead specimens, preserved in formalin or alcohol, or put up as permanent preparations for the microscope.

Definitions.

Colony, as used in this group, a number of individuals descended by budding from an original one, and remaining connected.

Polyp, an individual coelenterate; one of the individuals in a colony.

Observations.

1. How large is an individual specimen in the form you are studying? If the form is colonial, how large is the colony or portion of a colony you are studying? Estimate the number of individuals in it. Is the colony free-swimming or attached? If attached, to what is it usually fastened?

2. Compare the individual you are studying with a hydra, as to size and shape of the body, the location of the mouth, and the size, number, and arrangement of the tentacles.

3. Is there a skeleton? If so, describe it. What appears to be its use? In corals, notice the radiating partitions.

4. Has the specimen any nettle cells? If so, where are they located?

5. Are all the polyps of the colony alike? If not, how many kinds are there? How do they differ?

What is each kind best fitted to do? What is the probable result of this differentiation?

6. What kinds of reproduction, if any, does the specimen you are studying show?

Find out from books what other forms of reproduction are sometimes used by this animal.

Suggested drawings.

a. At least one drawing of each coelenterate you study.

Summary of the Comparative Study of Coelenterates

1. How may polyps in colonial forms differ from polyps which live singly?

2. What variations in methods of reproduction are shown in this group?

3. Which of the polyps you have studied shows the greatest differentiation? In what ways?

4. What characteristic do you find common to all the coelenterates you have studied?

Review and Library Exercise on Coelenterates

1. What are the characteristics which distinguish coelenterates?

2. Give the classes of coelenterates, with the characteristics and an example of each.

3. What enables a hydra to stick to a support by its foot?

4. What are the processes in a hydra by which food is captured, swallowed, and digested?

5. What is the chief fact of interest about Hydra viridis?

6. Why do hydras reproduce all summer by budding and in the late fall by eggs?

7. What change would have developed a hydra and its offspring into a plant-like colony instead of into a group of individuals?

8. Why are ctenophores more easily seen in the night than other coelenterates are?

9. What relations may exist between hydroids and hydro-medusæ?

10. What are the advantages of a sedentary life? Of a locomotory one?

11. What is meant by the expression "alternation of generations"? Which animals are likely to develop alternation of generations, sedentary ones or locomotory ones? Why?

12. Give at least two differences between hydro-medusæ and true jellyfishes.

13. In the association between a hydractinia colony and a hermit crab, what advantages are derived by the hydractinia? by the crab? Define symbiosis. Give another illustration of it.

14. How are new coral colonies started? How are large colonies formed?

15. What are the conditions of life under which corals can grow vigorously?

16. Where are corals most abundant?

Note.--Show by coloring the regions on a blank map of the world.

17. How may corals form a reef? Why do they, as a rule, form a reef instead of adding directly to the mainland?

18. Give Darwin's theory regarding the way a coral atoll may have been formed.

19. Where are fossil corals found in abundance? What does their presence prove?

20. What is polymorphism? Give an illustration. What may be a disadvantage of polymorphism? What may be an advantage?

21. In what ways is this group of economic importance?

4. A STUDY OF WORMS

To show cells associated even more closely than in coelenterates, forming tissues and systems of organs.

#A STUDY OF EARTHWORMS#

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.

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. 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. 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.

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 setæ 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 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.