4. A STUDY OF WORMS
To show cells associated even more closely than in cœlenterates, 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.
- 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?
- 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?)
- 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?
- 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.
- 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?
- 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?
- Where do earthworms live? What conditions are necessary in their habitat?
- When do earthworms usually leave their burrows? Why at that particular time rather than at another? Why does "the early bird catch the worm"?
- What enemies do earthworms have? How are they protected against these enemies?
- If you have found egg capsules when collecting worms, describe them.
External Morphology of Earthworms
Materials.
Preserved earthworms, the larger the better.
Observations.
- In what respects are the dorsal and ventral surfaces alike? In what respects different? Why?
- Why are the right and left sides alike?
- In what respects are the two ends alike? In what different? Why?
- 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?
- Where are the setæ located in a somite? How are they distributed over the body?
Suggested drawings.
- An earthworm, dorsal aspect.
- An earthworm, ventral aspect.
- 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.
- What is the extent of the body cavity, anteriorly and posteriorly? What is its shape?
- What, in general, is the shape of the food canal? How many external openings has it?
- Into what regions is the food canal differentiated? Suggest one advantage of having these specialized regions.
- How is the alimentary canal of the worm kept away from the body walls? Why have it thus supported?
- What is a septum? How many septa are there? What vessels and tubes pass through a septum?
- 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.
- 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.
- 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?
- 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?
- Notice the cells in the walls of the alimentary canal. What layers do you find? How are they arranged?
- 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?
- Study the cells of the nerve cord. How do they compare in size and shape with the muscle cells?
Suggested drawings.
- A diagram of a cross section, showing the relation of the organs.
- A diagram of a longitudinal section, at least through the body wall, to show the arrangement of muscle fibers.
- A drawing of a portion of the body wall, to show details.
Summary of Important Points in Study of the Earthworm
- Compared with a hydra, how many cells has an earthworm?
- Compared with a hydra, how much are the cells of an earthworm differentiated?
- How are these differentiated cells usually arranged with respect to one another? What advantage is there in this arrangement?
- 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.
- How large is the specimen and what is its shape?
- Can you distinguish a head or a head end? If so, by what peculiarities?
- 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.
- State whether the animal is bilaterally symmetrical, radially symmetrical, or without symmetry.
- Compare this worm with the earthworm as to sense organs.
- What organs for respiration has it?
- What special protective devices has it?
- 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
- Name the different worms you have studied. What characteristics have they in common?
- What different methods of obtaining food do they show?
- What variations do they show in senses? in sense organs?
- Which one seems to you best adapted to its habitat? In what ways?
Suggested drawings.
- One drawing of each worm studied.
Review and Library Work on Worms
- What are the distinguishing characteristics of worms?
- Give the classes of worms, and the authority for this classification.
- 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?
- In what ways do earthworms benefit the soil? How great is their effect estimated to be?
- 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?
- How are earthworms protected against the cold of our winters? What limits the northern range of earthworms?
- Where are earthworms found geographically? Why are they so widely distributed? By what means are they extended from one locality to another?
- How do earthworms reproduce? What care do they take of their young?
- 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?
- 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?
- Contrast a cross section of hydra with one of earthworm as to the number of cavities. Which seems to you the better plan? Why?
- Why does a nereis need more respiratory surface than an earthworm does?
- Comparing earthworm and nereis, in what respects is the earthworm degenerate? How does it manage to succeed so well with such a degenerate body?
- What is a parasite? How many hosts does a typical parasite require for its development? Which host is known as the intermediate one?
- 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?
- 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?
- Why do worms so easily become parasitic? What advantages are there in becoming a parasite? What disadvantages?
- 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?
- 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?
- Name various methods of locomotion among worms. Give examples. Name a fixed or sedentary worm.
- 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, cœlenterates, and worms. Write the results in the following summary:—
- What work, i.e. labor, must an animal do to live?
- How many cells are necessary to do this work?
- When this work is divided among a number of cells, what is the effect upon the quantity and quality of work accomplished?
- 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.
- 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.
- Is division of labor a good thing for an animal body, or is it not? Give reasons for your opinion, with examples for illustration.