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.

1. An earthworm, dorsal aspect.
2. An earthworm, ventral aspect.
3. 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

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.

1. Earthworm, showing structures mentioned in this study.

Details of Structure—Microscopic Anatomy

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.

1. A diagram of a cross section, showing the relation of the organs.
2. A diagram of a longitudinal section, at least through the body wall, to show the arrangement of muscle fibers.
3. A drawing of a portion of the body wall, to show details.

Summary of Important Points in Study of the Earthworm

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.

1. 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

CHAPTER IV
ADAPTATION TO SURROUNDINGS

Living Crayfishes

1. Observe living crayfishes in their usual habitat or in a large aquarium, without disturbing them, and see where they stay when they are free to choose. Notice their position. What senses are on guard? What is the color of the head and claws? How may this color aid the animal in getting food or in escaping enemies? Why is the color of the posterior region less important than the color of the anterior?
2. Offer them bits of meat. If one takes food, notice the appendages it uses. How does it discover the food? With what appendages does it grasp the food? How is the food conveyed to the mouth? With what senses, if any, does the animal test the food as it eats it?
3. If the crayfishes are in plenty of water and you startle them in any way, some of them may swim. Watch for such an occurrence and notice it carefully. How is swimming accomplished? Which end leads in swimming? How far does the animal swim at a stroke? How long does it continue to swim? Where does it go? Does it see where it is going? For what purpose would this method of locomotion be useful?
4. Place a living crayfish in a tray with water to cover it, and take it to your table. Watch the crayfish as it walks about in the water, then take it out and let it walk out of water. Compare the two processes. What causes the differences?
5. How many appendages are used in walking? What order, if any, is there in moving the legs? Which method, walking or swimming, does it use in going to some particular spot, e.g. in going to find food or cover? Why?
6. Gently turn the animal on its back and watch the movements of its appendages as it rights itself. Which appendages does it use and how does it use them? How can it manage to use so many appendages in harmony, for one result?
7. For what different purposes have you seen the crayfishes use their large claws? For which does the claw seem best fitted? Can you think of any change which would make it more efficient for its main purpose? If so, describe the change and tell how it would work.
8. Test the distribution of the sense of feeling. Is it anywhere especially acute? If so, where? Why have two pairs of feelers? Where is each pair carried when the animal is at rest; when it is in motion? How much territory can the two pairs guard?
9. Touch the eyes. Compare their sensitiveness with that of your own eyes. What movements can the eyes perform? How are they protected? What range of territory can they guard?
10. What other senses, if any, do you think a crayfish has? Why do you think so?
11. Early in the spring crayfishes may be found carrying eggs or young. If such a specimen is at hand, notice where and how the eggs or young are attached. How many are there? How are they cared for? Can the young crayfish let go? If removed, can they attach themselves again? How much care does the mother give them when they are removed?

Morphology of a Crayfish

Cephalo-thorax,

the anterior half of the body, divided into the head and the thorax.

Cervical groove,

the groove dividing the head from the thorax.

Abdomen,

the posterior half of the body, consisting of a number of somites.

Note.—The central part of the tail fin is usually included as a somite.

Carapace,

the continuous shell-like portion of the exoskeleton covering the cephalo-thorax.

Rostrum,

the sharp projection of the carapace at the anterior end.

Gill chamber,

a pocket on each side of the thorax, covered by a flap of the carapace.

Appendages,

paired structures attached to the body. They are named as follows:—

Eyestalks.

(These are not classed as appendages by all students.)

Antennules,

the small feelers.

Antennæ,

the large feelers.

Mandibles,

the jaws, one on each side of the mouth.

Maxillæ,

the two pairs of small mouth parts just back of the mandibles.

Maxillipeds,

the three pairs of appendages between the maxillæ and the large claws.

Chelipeds,

the large claws or pinchers.

Walking legs,

the four pairs of appendages back of the chelipeds.

Swimmerets,

the appendages on the abdomen.

Openings,

five on the ventral surface, as follows:—

• The openings from the excretory organs, through small white cones on the bases of the antennæ.
• The mouth, farther back, between the maxillipeds.
• The anal opening, in the last segment of the abdomen.
• The opening from the reproductive gland, toward the posterior part of the thorax.

Observations.

1. How large is your specimen? How does it compare in size with other crayfishes in the laboratory?
2. Describe the shape of the body, contrasting the anterior end with the posterior, and the dorsal surface with the ventral.
3. Study the amount of motion permitted in different parts of the body. What prevents motion? What permits it? Where is the body most flexible? Why? Where is it most rigid? Why?
4. How much of the surface is covered with exoskeleton? What arrangement is there to permit the animal to feel contact?

   How can the animal grow with such an exoskeleton?

5. Place a dead crayfish in dilute acid for a few hours. What is the result? What has the acid done? Explain the fact that crayfishes are often found alive and well with a soft shell?
6. Compare the cephalo-thorax with the abdomen as to size, shape, and flexibility.
7. How many somites are there in the abdomen? Which way does it bend? Study the somite shells on every side and then state what there is in their construction which determines the direction and amount of their motion. How are the somite shells arranged to protect the body during bending? How is the ventral surface of the abdomen protected?
8. Where are the appendages attached? Study a walking leg and describe its general construction, the number and kind of joints, the direction of motion in each joint, and the range of motion for the whole leg. Study an antenna in the same way. What methods are used in the crayfish to secure a wide range of motion? To secure flexibility?
9. Carefully split a crayfish into right and left halves. To do this, first cut through the ventral exoskeleton from end to end with scissors, then with a sharp knife or razor cut through to the dorsal exoskeleton and cut that with the scissors. Study one half, to get a better idea of the attachment of the appendages. These may then be removed and placed in order on a piece of paper upon which a list of the appendages has been written.
10. How many pairs of appendages are there? How may they be grouped according to location; how grouped according to function? How many pairs are there in each group?
11. What similarities of structure do you find in nearly all of the appendages? Assuming a swimmeret of the third, fourth, or fifth somite to be the least changed from the primitive type, what changes were necessary to make the sixth swimmeret; the third maxilliped; the walking legs; the antennæ; the antennules?
12. Remove the part of the carapace which covers a gill chamber. What are the boundaries of the chamber? Where does it open to the water?
13. Describe the appearance and the texture of a gill. How are the gills kept moist when the crayfish is in water; when it is on land? Why should they be kept moist?
14. Would you class the gills as external structures or as internal? Why do you think so? To what are they attached? How are the gills affected by the motion of the legs?
15. What work goes on in the gills? How is the supply of oxygen renewed? In this connection, try a live crayfish, kept quiet in water just about deep enough to cover it. Float bits of paper near it or carefully place a drop of ink in the water near it. By some such method currents of water may usually be shown, and their direction determined. Consider also the habitual motions of mouth parts and swimmerets, the bubbles sometimes seen when a crayfish is dropped into water and the habit crayfishes have of lying on one side, close to the surface of the water.

Summary of the Study of Crayfishes

1. What are the varying conditions in their surroundings which crayfishes must meet? Which are most important?
2. What conditions must be maintained in order that crayfishes may succeed, i.e. may live and reproduce?
3. How does a crayfish know what are the conditions around it?
4. How is it fitted to meet these conditions? (Answer in the following details):—
   1. How wide a food range has it, i.e. how many kinds of food does it eat? How does it find its food? How does it reach it? How does it take its food? How does it make food small enough to be eaten?
   2. What are the organs for taking in oxygen? Where are they? How are they attached? How is the supply of oxygen kept up? How are the organs kept from drying, from clogging, and from mechanical injury?
   3. What ranges of temperature can crayfishes endure? What temperature is best? How do they avoid fatal extremes?
   4. What are the enemies of crayfishes? What protection against these have they?
   5. How often do crayfishes reproduce? About how many times during a normal lifetime? About how many eggs are there and how many of them hatch? What care is given to the eggs and to the young? About how many of the young reach maturity? (Suggestion. Do the crayfishes of a region vary noticeably in numbers from year to year?)
5. What limits the range of crayfishes, north and south? What limits it on land? What in water?
6. When the crayfishes of a given locality are not well adapted to it, what can they do?

Suggested drawings.

1. The whole animal, dorsal surface, preferably without appendages.
2. One of each pair of appendages, except where they duplicate.
3. The tail-fin. Label the sixth swimmerets, the sixth and seventh somites.
4. The gill chamber, with gills in position. Show circulation of water by arrows.
5. A gill, to show construction.

2. COMPARATIVE STUDY OF CRUSTACEA

Materials.

Get together all the different specimens of crustacea you can collect, and identify the material. Then study each specimen as follows:—

Questions.

1. Briefly describe the exoskeleton, if there is one.
2. What region or regions are clearly segmented?
3. How much of the body is covered by a carapace?
4. Has it segmented appendages? Classify the appendages as to their use.
5. Are the cephalo-thorax and abdomen equally developed? If not, which is more developed?
6. How many antennæ has it? Are the eyes stalked, or are they sessile?
7. What organs of respiration has it? Where are they attached?
8. How many thoracic appendages has it, if any?
9. What methods of locomotion does it use?

Summary.

1. Does this animal seem to be adapted to life on land or in water, or both? Give your reasons for your opinion.
2. What characteristics are common to all the crustacea you have studied?

3. REVIEW AND LIBRARY WORK ON CRUSTACEA

1. What are arthropods?
2. Give the classes of arthropods with an example of each.
3. What are the distinguishing characteristics of the class crustacea?
4. In what respects are most of the appendages of the crayfish homologous, i.e. alike in the plan of structure? Which do you consider the simplest, and why do you? Which do you consider the most specialized, and why?
5. Which somite of the crayfish is without appendages? How many somites are there in a crayfish's body, if each somite bears only one pair of appendages, as many scientists believe? How many of these are in the head; thorax; abdomen?
6. Compare the nervous system of the crayfish with that of the earthworm as regards efficiency. Upon what do you base your answer?
7. Name two points in which earthworms and crayfishes are alike. Name three in which they differ.
8. How are crayfishes caught for market? Where do those sold in Chicago usually come from? How are they shipped?
9. Compare the young forms of a crayfish and a crab.
10. Describe any five different crustacea.
11. Describe the work done by the United States government and by the state governments to protect and to perpetuate the lobster. Why is it thought necessary to do this?
12. Discuss the process and the advantages and disadvantages of molting, as seen in the crustacea.
13. Name two advantages in having such a shell as crustacea have. Name two disadvantages. On the whole, is such a shell favorable to an animal's chances of success or is it not?
14. Give the curious myth about goose barnacles.
15. What crustacea are parasitic? Give an account of one.
16. Why are barnacles classed among crustacea? Where were they once classed? Why may they be considered degenerate, even though not parasitic? How do they manage to succeed? What is their economic importance? How are their effects checked or prevented?
17. Describe some of the odd means of self-protection shown among crustacea.
18. Describe a compound eye. Give two theories as to what can be seen with a compound eye. Why do we not know, instead of theorizing?
19. What is the economic value of the very small crustacea?
20. Discuss the value to man of the various forms of crustacea.

CHAPTER V
ADAPTATIONS FOR PROTECTION FROM ENEMIES

1. THE CLAM—A TYPE OF MOLLUSCA

Mollusca,

a branch of the animal kingdom including those animals with soft, unsegmented bodies, inclosed in two folds of skin known as the mantle. They are often called shellfish as most of the forms have a shell.

Lamellibranchiata or Pelecypoda,

names given to the class of Mollusca to which the clam belongs. The former term refers to the broad, flap-like gills and the latter to the hatchet-like foot.

Valve,

one of two parts of the clamshell.

Hinge ligament,

the elastic structure which fastens the valves together at the dorsal margin.

Umbones,

a pair of elevations near the anterior end of the shell.

Lines of growth,

concentric lines around the umbones.

Siphons,

two openings at the posterior end of the clam, the upper opening is the excurrent opening and the lower the incurrent. In the salt water clam the siphons form a long tube, usually called the "neck."

Hinge teeth,

projections near the dorsal margin on the inner surface of the shell. The anterior irregular structure is the cardinal and the more posterior blade-like structure is the lateral tooth.

Muscle scars,

scar-like markings on the inner surface of the shell indicating the point where muscles were attached. The large scar just in front of the cardinal tooth is the anterior adductor muscle scar, and the one just back of the lateral tooth is the posterior adductor muscle scar.

Pallial line,

a line connecting the two muscle scars.

Mantle,

folds of skin covering the body of the clam and lying close to the inner surface of the valves.

Foot,

a hatchet-shaped structure extending from the ventral edge of the body.

Gills,

broad flap-like structures for respiration, situated each side of the body in the mantle cavity. They consist of a double fold of membrane through which run many perforations lined with cilia. The waving of these cilia cause the current of water needed for respiration.

Palps,

small flap-like structures near the anterior end of the clam. They surround the mouth. On their surface are cilia which cause currents of water toward the mouth.

Adductor muscles,

large muscles extending from valve to valve.

Observations.

Identify anterior and posterior ends, dorsal and ventral surfaces, right and left sides.

1. Why may a clam be called a bivalve?
2. What is the position of the clam in the mud? What is the position of the foot if the clam is undisturbed? Are the two valves tightly closed or slightly open at this time?
3. What changes take place in the shell as the clam grows? What markings on the surface of the shell indicate this?
4. Where is the clam sensitive to touch or tactile stimulus? Why has the clam no eyes? Zoölogists have found a structure in clams which they have supposed to be an ear. Where do you think the structure is located? Why is the clam successful without eyes? (There are many bivalves which have them.)
5. Examine several clams until you find some with enlargements in the gills. Break off a small part of an enlargement with your forceps and examine under the compound microscope. Describe what you see.
6. Drop some powdered chalk or carmine in the water just above the siphon, watch the siphons for several minutes, and note what happens. What do you conclude to be the use of the siphons? Recalling what took place in sponges, what would you suggest as the probable cause of these currents? What does the clam thus probably obtain? How do the two siphons differ? Why?
7. Place a clam in water sufficient to cover it and heat slowly to about 40 degrees Centigrade, until the valves open slightly. Remove and proceed as follows: Raise one valve, separate the mantle from it, and then cut through the two large firm structures (adductor muscles) found at each end. What does the valve do when the muscles are cut? What is the cause of this? State your theory as to how a clam opens and closes its shell.
8. Note the texture of the mantle. How many lobes has it? What is their extent? How are the lobes related to the valves?
9. Remove or lift up one mantle lobe. Identify the soft body, the foot, the gills, the palps, and the mouth. Which of these structures are arranged in pairs?
10. Determine the structure and composition of the shell as follows:—
    1. Break a thick clamshell and examine the broken edge. Identify the inner or pearly layer and the outer or chalky layers. What gives color to the shell in the living clam?
    2. Burn a small piece of shell in an evaporating dish over a bunsen burner. What is the appearance of the shell after burning? What has been burned, animal or mineral matter? What then is the residue?
    3. Place a small piece of shell in acid. What results? Is there a large amount of residue? What constitutes the greater part of the shell, animal or mineral matter?
    4. (Optional) Devise some method and determine the approximate per cent of mineral and of animal matter in the clamshell.

Summary.

1. Why did we study the clam? (See title of exercise.)
2. How has the heavy shell of the clam affected:—
   1. The character of the clam's body,
   2. the locomotion,
   3. the development of sense organs.
3. What special problems has the clam as regards getting food and oxygen? How are these problems solved?
4. How does the clam protect the young clam during development?

Suggested drawings.

1. Dorsal margin of the clam.
2. Side view of the clam.
3. The clam with one valve removed or lifted back.
4. The clam with one valve and one mantle lobe removed.
5. The edge of a broken shell.
6. Diagram of cross sections.

2. THE SNAIL—A TYPE OF MOLLUSCA

1. Why is a snail called a univalve?
2. Identify the head and mouth of the snail. Watch the snail feeding and examine the mouth of the snail with a lens. What do you notice? If your aquarium in which the pond snail is living has a green coating (algæ) on the side, describe its appearance after the snail has been crawling up and down over it. Explain.
3. How many tentacles has a pond snail? a land snail? Where are the eyes located in each case? What movements of the tentacles do you notice? What is their purpose?
4. How does the rate of locomotion of the snail compare with that of the clam? Find out if the snail can creep backwards or on the surface of the water. Does there seem to be any tendency for the snail to go up and down the sides of the aquaria vertically rather than to the right or left?
5. What does a snail do when disturbed? What is gained by this action?
6. Search for pond snail's eggs on the side of the aquaria. Lift up the bits of cabbage on which the slugs are feeding and search for eggs. Describe what you find in each case, noting the size, appearance, and whether the eggs are laid singly or in masses.
7. Find the breathing pore. Describe its position and appearance.
8. Contrast the various types of shells, and note with care in what respects they differ. Holding the shell with the aperture toward you and the spire pointing up, determine whether each shell has the aperture on the right (right-handed shell) or on the left (left-handed shell). Is the right-handed or the left-handed shell more common?
9. (Optional) By means of some book in the laboratory, determine the scientific name of each of the snails found in the various aquaria in the laboratory.

Suggested drawings.

1. Drawings to show the pond snail in various positions in the aquarium.
2. A drawing of the slug.
3. At least three different types of snail shell.

Summary.

1. In what respects does a snail show resemblance to a clam?
2. What are the chief points of difference?
3. What reasons can you suggest for the better development of the sense organs?
4. What advantage has a snail over a clam in the matter of getting food?
5. How does the shell of the snail compare with that of the clam as an organ for protection?

3. THE SQUID—A TYPE OF MOLLUSCA

Cephalopoda,

the name of the class to which the squid belongs.

Caudal fin,

a horizontal structure at the posterior end of the squid.

Chromatophores,

irregular cells in the mantle which give color to the squid.

Exhalent siphon,

a funnel or tube opening on the ventral side just below the base of the arms or tentacles.

Pen,

a remnant of an exoskeleton imbedded in the mantle along the dorsal side.

Ink sac,

a sac containing a dark, sticky liquid which may be thrown out through the funnel into the water. The opening is near the inner opening of the funnel.

Observations.

1. What is the shape of the squid? To what is this shape adapted?
2. Identify the head and the well-developed eyes.
3. How many arms or tentacles are there? How are they arranged with reference to the mouth? What do you find on the distal ends of the arms? How do the arms vary as to size? What does the position and arrangement of the arms suggest as to their function?
4. Identify the exhalent siphon. Where may water enter the mantle cavity? Recalling the action of the siphons in the clam, suggest a method by which a squid is propelled through the water. In what direction must it swim?
5. Split the mantle along the ventral surface and spread apart. Identify the long plume-like gills, the ink sac, and the inner opening of the exhalent siphon. How many gills do you find?

Suggested drawings.

1. The squid side view.
2. The squid from the ventral side with the mantle split open, arrows to show direction of water.

Summary.

1. In what ways does a squid show relationship to the clam and the snail?
2. What has a squid gained through the reduction of its exoskeleton? What has it lost? What changes were necessary in its structure to offset the loss of an exoskeleton?

4. A COMPARATIVE STUDY OF MOLLUSCA

1. What is the symmetry?[3]
2. Is the body segmented or unsegmented?
3. Are lateral appendages present or wanting?
4. Is an exoskeleton present or wanting? If present, is it univalve or bivalve; if absent, what other means of protection has been developed to take its place?
5. Is the animal fixed, or is it free to move? If fixed, in what way? If it moves, what is the method and organ of locomotion?
6. What are the organs of respiration? What is their character?
7. How is food obtained?
8. What senses are probably present? What sense organs are present?
9. What is the habitat?
10. In what ways if any does the animal show degeneration?

Summary.

1. What characters are common to all mollusks?
2. What is the principal means of protection among mollusks?
3. Name three causes of degeneration among mollusks.

5. MOLLUSCA: REVIEW AND LIBRARY EXERCISE

1. Oyster culture.
2. Typhoid-fever and oysters.
3. Clams, scallops, and other edible shellfish.
4. Pearls and pearl fisheries.
5. Fresh-water clams and the button industry.
6. Sepia, Tyrian dye, etc.
7. Harmful and useful mollusks.
8. The work of U. S. Fish Commission in propagating clams.

Natural history.

1. Give the class, habitat, and some important fact about each of the following: Pectens; wing shells; Tridacna gigas; abalones; limpets; oyster drill; periwinkle; mussel; cuttle fish; octopus; nautilus; argonaut.

6. A COMPARATIVE STUDY OF EXOSKELETONS

1. What are foraminifera; radiolaria? How do they differ from other protozoans? Of what two substances are the shells of protozoans composed?
2. How are the spicules formed in a simple sponge? What are glass sponges? Give reasons why the skeletons of sponges may or may not be considered exoskeletons?
3. What are stone corals? What is the relation of the coral polyp to the skeleton? What is the appearance of the coral when expanded as compared with its appearance when contracted? Of what substance is the coral composed?
4. Describe the exoskeleton of a starfish. Contrast the exoskeleton of the sea urchin and the starfish. Why does a sea cucumber need no well-developed exoskeleton?
5. What structure in an earthworm may be considered an exoskeleton? What other types of exoskeletons are found in segmented worms?
6. Of what substance is the exoskeleton of arthropods composed? What additional substance is found deposited in the shell in the case of crustaceans? What advantage in the arthropod type of exoskeleton?
7. Why are mollusks so commonly called "shellfish"? What advantage in the mollusk type of skeleton? What disadvantages?

Summary.

1. What type of exoskeleton is common among invertebrates?
2. What are the general purposes of exoskeletons?
3. What is the explanation of the various forms of exoskeletons found?
4. Of what substances are exoskeletons composed?

B. Protective Coloration

General protective resemblance,

the general resemblance between the color of an animal and its surroundings.

Variable protective resemblance,

the changing of the color of an animal to correspond to the change in its background.

Special protective resemblance,

the resemblance of an animal to some object found in its background in color and form.

Mimicry,

the resemblance of an unprotected animal to a well-protected one.

Warning colors,

bright colors which protect animals by causing other animals to avoid it.

Questions.

1. Show how the transparent color of a paramecium, the green color of a cabbage worm, or the green color of a certain species of hydra may result in protecting an animal from its enemies. Mention as many other examples as you can.
2. What is gained by the ability of a squid to change its color? How is this change brought about?
3. Explain the protective coloration of the following: Dead-leaf butterfly, walking stick, geometrid larva. Hunt up other examples.
4. Explain the protective coloration in the following: Hover flies, clear-winged moths, viceroy butterflies.
5. Make a list of several invertebrates that are protected by their bright color. Explain the reason for the bright color.
6. How may the difference between the color of the upper and lower surfaces of animals be explained on the basis of use to the animal?
7. (Optional) Find out some other uses of color to an animal aside from protection.

Summary.

1. Name four uses of color.
2. Name four ways an animal is protected by being like its background.
3. Name one way it is protected by being unlike its background.
4. What disadvantages in this method of protection?

C. Animal Associations

1. Examine a specimen of Volvox. Why may this be considered a colonial protozoan and not a many-celled animal? What is gained by the colonial habit?
2. Is the colonial habit common or rare in sponges and cœlenterates? What is chiefly gained?
3. Describe the community life in one of the insects in each of the following groups:—
   1. ant, honeybee, termite.
   2. bumblebee, paper wasp, hornet.
   3. mining bee.
   4. carpenter bee, mud wasp, digger wasp.
4. Name the host or hosts in the following cases: trichina, liver fluke, malarial parasite, tapeworm, hook worm. Give the life history of one or more of the parasites just enumerated. What is the effect of parasitism on the structure of the parasite?
5. What is the relation between ants and plant lice? Show how this relation is mutually helpful. Mention other cases of symbiosis that you have come across.
6. With what animal are barnacles often associated? What is the habit of the pea or oyster crab? What are "guest bees"? What structure is lacking that is found in other bees? What are often found in the cavities of sponges? Why are these associations called commensalism rather than symbiosis?

Summary.

1. Into what groups can animal associations be divided based upon the number of species concerned?
2. From the standpoint of protection, is this a good or a bad method of protection?
3. What disadvantages can you see in this method of protection.

D. Protective Habits and Powers

E. Defensive Structures

F. Thesis

1. The struggle for existence—
   1. its cause,
   2. its threefold nature,
   3. the various kinds of adaptations.
2. The various methods of protection from enemies.
   1. The exoskeleton.
   2. Protective coloration.
   3. Animal associations.
   4. Protective habits.
   5. Defensive structures.

CHAPTER VI
VERTEBRATES

THE LIVING FISH

Trunk,

the portion of the body between the head and the tail.

Compressed,

a term used to describe the shape of the body when it is narrower from side to side than from dorsal to ventral surface. When the opposite is true, the body is said to be flattened.

Median fins,

the unpaired fins situated on the median line, dorsal and ventral, including the tail or caudal fin, the dorsal fin, and the anal fin.

Paired fins,

fins occurring in pairs of which the more anterior are the pectoral fins and the posterior are the pelvic fins.

Fin rays,

the framework or skeleton of the fins over which membrane is stretched to form the fins. Fin rays are of two kinds: those composed of bone and those composed of cartilage.

Lateral line,

a sense organ extending along each side of the fish in a line indicated by tubes or perforations in the scales.

Gills,

respiratory organs adapted for taking oxygen from the water.

Operculum,

the flaps covering the gills on each side of the head.

Pigment,

a substance which gives color to an object.

Observations.

Locomotion.

1. Watch the fishes in the large aquarium and determine which fins are most used and how they are used (a) in swimming forward, (b) in swimming upward and downward, (c) in maintaining balance, (d) in remaining at rest, and (e) in guiding the movements of the fish.
2. What advantages are there to the fish (a) in the power to open and close the dorsal and anal fins, (b) in having no neck, and (c) in having a compressed form?
3. Enumerate the various ways by which the body of the fish is adapted to rapid movement through the water.

Feeding.

1. What is the food of the fishes you are studying? Feed them and watch them eat. Why is the upper jaw often called a "lip"? What is the shape and size of the mouth when opened in feeding? Does the fish chew its food? Describe in detail the fishes' method of feeding.

Respiration.

1. Identify the opercula and the gill openings. Watch the movements of the opercula and mouth, and determine what movements are concerned in breathing and their order. Describe in detail the circulation of water used in breathing and how it is caused.

Sense Organs.

1. Identify the eyes, nostrils, and lateral line. How many nostrils are there and where located? What is the position and extent of the lateral line?
2. Describe the location of the eyes. What is the shape of the outer surface of the eyes? Why this shape? Can the eyes be moved, i.e. can they be rotated, rolled, or retracted? From what direction might an enemy approach without being seen? How would such an enemy be detected?

Protection.

1. With what protective structures is the body covered? Do they hinder the movements of the fish? What are the advantages of the scale covering of fishes over the shell covering of grasshoppers or crayfishes?
2. In what other ways are the fishes you are studying protected against enemies? Since you cannot account for the red color of goldfishes on the basis of use to the fish, then how do you account for this bright color?

The Body.

1. What is the symmetry of the fish? Into what regions is the body divided?

Summary of the study of the living fish.

Enumerate in one column the different adaptations which fit the fish for life in water and in a second column state the special purpose of each adaption.

The External Structure of the Fish

Examine the fins and identify the membrane and the supporting rods, or rays, of bone or cartilage. Notice how the ends of the cartilaginous rays keep the membrane from tearing.

Investigate the scales as to their arrangement, number, and size. Remove a small patch of scales along the lateral line to find how they are attached, where the fish's color is situated, and how access to the sensory organs of the line is permitted. Examine a scale under the microscope.

Observe the eyes and identify the parts similar to those of the human eye: lid, lash, tear-duct, cornea, iris, and pupil.

In front of and between the eyes, find the nostrils. By means of a bristle determine whether these are connected and whether they do or do not open into the mouth or the throat.

Questions.

1. Make a list of the fins, classifying them according to their structure.
2. Bearing in mind the differences in structure and consequent action,—what can you say regarding the adaptation of the several fins for protection? for rigidity or flexibility in locomotion?
3. State how much of the body is covered with scales, and where the largest and the smallest ones are found.
4. How are the scales arranged with reference to each other? What benefit is derived from this in protection? in locomotion? If you have noticed any mucus or slime upon the body, state its use.
5. Do the scales or the skin bear the pigment? Give the color pattern of the kind of fish used in class. How would this be useful to the fish in its natural home?
6. Describe the structure of a scale and state how it is attached to the skin. In what way is the lateral-line scale specialized?
7. State how, when the fish is swimming, the nostrils catch odors. By means of a diagram, with arrows show the probable direction of the water current through the nose.
8. State which of the structures of your eye are present in the fish's eye, and which are missing. Could a fish weep? wink? How would a fish sleep?
9. Inasmuch as light penetrates water but a little way, so that objects can be distinguished only within about thirty feet, would the fish be nearsighted or farsighted?

Suggested drawings.

1. A side view of the entire fish, fully labeled.
2. A bony rayed and a cartilaginous rayed fin.
3. A scale, showing its minute structure.
4. A dorsal or a lateral view of the head, showing the sense organs.

The Mouth and the Gills of the Fish

The mouth may be kept open by a short splinter or a ball of paper.

The pupil should identify the following structures:—

1. Gill,

an organ for breathing the air dissolved in water.

2. Gill arch,

an arch of bone or cartilage supporting the gills.

3. Gill filaments,

fringe-like structures attached to the gill arches, forming the gills.

4. Gill raker,

lateral projections from the gill arches.

5. Gill-slits,

openings between the gill arches for the passage of water.

6. Operculum,

the flap-like covering of the gills on each side of the head.

Questions.

1. Compared with the size of its body, how wide can the fish open its mouth? What do you infer as to the size of its "bite"?
2. Are the jaws rigidly affixed to the skull? Why should they be so attached, or why not?
3. Of how many pieces is the upper jaw composed? the under jaw?
4. Where are the teeth? Judging from their form, size, and situation, what do you think must be their use?
5. Do you think the tongue is used to assist in mastication? in tasting? in speech? in swallowing?
6. How many gills are there, and where are they situated? How are they attached? Which one is not free from the body throughout its length?
7. What probably causes the color of the gill filaments? What is there in their number and texture which fits them for their function?
8. What is the direction of the water current through the gill chamber? Of what use are the gill rakers?
9. How are the gills protected?

Summary.

Write a complete account of how the fish eats and how it breathes.

Suggested drawings.

1. A front view of the fish's face, with the mouth fully open.
2. A side view, as above.
3. A ventral view of the head, with both gill-chambers wide open and the gills separated from each other. Indicate currents by arrows.
4. A single gill.

The Alimentary Canal and the Circulatory System of the Fish

1. Body cavity, the entire internal space, divided by a membrane, false diaphragm, into a large abdominal cavity and a small chamber, pericardial chamber, between the gill chambers.
2. Liver, a large red or pink mass lying at the front end of the abdominal cavity, and divided into two unequal lobes. The gall-bladder, thin-walled and green, may be seen between these lobes.
3. Alimentary canal.
   1. Mouth.
   2. Esophagus, in the fish a very short tube.
   3. Stomach, white and muscular, beginning with a very short esophagus and ending as a blind sac. If it is much distended, open it to see what the fish may have eaten.
   4. Small intestine, thin-walled, tubular, and somewhat coiled.
   5. Large intestine, a short, thin-walled expansion at the posterior end of the small intestine; usually less than half an inch long.
   6. Cœca, from two to several small pouches attached where the small intestine leaves the stomach.
4. Spleen, a reddish brown globule between the folds of the intestine.
5. Swim bladder, an elongated chamber lying against the backbone, partitioned off from the cavity below by a delicate membrane.
6. Peritoneum, the delicate, silvery membrane which lines the abdominal cavity and enfolds the viscera. Note its spots of pigment.
7. Pericardial chamber, the chamber around the heart; see § 1 above.
8. Heart. As the fish is placed belly upward in the pan the ventricle faces you, pink, conical, and muscular. Posterior to it, on the dorsal side, is the auricle, a membranous sac.
9. Ventral aorta, arising on the anterior surface of the ventricle as a white muscular "cord" (really a tube) which is enlarged close to the heart into a bulb, the arterial bulb. You should follow up this aorta until you see it divide right and left to send its branches outward into the gills, the branches being called gill arteries.

Questions.

1. The fish frequently swallows its food alive. Why should the stomach be muscular? Why is it better that the intestine does not leave the stomach at the end opposite the esophagus?
2. Of what use can the cœca be? What structure of the human intestine do you recall that is at all like them in form or use?
3. How many times the length of the body is the length of the alimentary canal? Does this indicate that the fish is compelled to eat a great deal of poor food or that its food is highly nutritious, so that little need be taken?
4. Near which end of the fish's body is the heart? Is this the usual or the unusual condition among animals you know about? What advantages can you think of in this arrangement?
5. What advantages are there in having the heart in a chamber separated from the other vital organs?
6. Of how many chambers does the heart consist? Why should at least one of them be muscular?
7. How many times does the blood pass through the heart in making a complete circuit of the body? Would you call this a single or a double circulation?
8. Does the heart force the blood onward or does it draw blood into itself, i.e. is the heart a force pump or is it a suction pump?
9. How is circulation made complete? If the heart is a force pump, is its power sufficient to drive blood through artery, capillary, vein, and into auricle, if the capillaries can stand the pressure, or is another action concerned? If it is a suction pump, why does the blood leave the heart?

Suggested drawings.

1. The body cavity, with viscera undisturbed.
2. The alimentary canal extended.
3. The anterior end of the fish with the sinus held open, to show the general situation of the parts.
4. The heart in its chamber, with the outgoing vessels as far as dissected. Use arrows to show direction of circulation.
5. A copy of some good diagram or chart which illustrates the heart of the fish with the connecting veins and arteries.

Fishes: A General Review and Library Exercise

1. Food and the feeding habits of young and of adult fishes.
2. The diet and habits of cod; lantern-fish; swordfish; ramora; hagfish; angler; gar-pike; sturgeon; shark; sawfish; paddle-fish.
3. The variations, real or apparent, in the breathing habits of the porcupine-fish; the climbing-fish; the lung-fish.
4. Peculiarities in swimming as seen in the flying-fish; the flounder; the sea-horse.
5. Intensity of sound under water, and the corresponding structure of the fish's ear.
6. Light and sight under water (as in 5).
7. Protection of fishes: sting-ray; torpedo; coral-fish; sturgeon; lava-fish; swordfish; sawfish; pipefish.
8. The social instinct of fishes, and "schools."
9. The breeding habits of salmon; eel; stickle-back; sturgeon; whitefish; shark; sea-horse; sunfish.
10. The fishing industries of the Great Lakes or of the cold oceans, with a list of the fishes caught and their values.
11. Fish nets and traps: seine; gill-net; pound-net; trawl, French or English; fish-wheel; fish-weir; spear; dip-net; set-line; spoon; fly.
12. The U.S. Bureau of Fisheries: its locations, its problems, and its methods.
13. The State Fish Commission, as above.
14. Game and fish laws; their purpose and their enforcement.
15. Game fish of the fresh waters; trout, bass, pickerel, and muskellunge.
16. Game fish of the ocean: tarpon, tuna, sea-bass, swordfish, and bluefish.
17. Fish as food.
18. Fish diet and leprosy.
19. Fish diet and parasitic worms.
20. Fish nuisances: carp, catfish, and dogfish.
21. Commercial products of fishes, their preparation and their uses: caviar, shagreen, cod liver oil, isinglass, and glue.
22. The geographic distribution of fishes, with means of dispersal and restriction.
23. The faunal regions of the lake (or ocean), with characteristic forms.
24. Fishes of ancient times; of the Devonian period.
25. The story of the early life of Louis Agassiz; of D. S. Jordan; of C. H. Eigenmann; of Bashford Dean.
26. Goldfish: their origin; how to care for them.
27. Fashions in fish tails, old and new.
28. Development and variation in scales; fashions in scales.
29. The common orders of fishes, with examples.

Primitive Chordates