A. The Exoskeleton

1. THE CLAM—A TYPE OF MOLLUSCA

To Show the Effect of a Heavy Exoskeleton

Materials.

Living clams in aquaria, with enough moist sand to cover the clams, preserved clams, sets of matched clamshells, a few shells with the hinge unbroken, evaporating dishes, hydrochloric acid.

Definitions.

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

To show Another Type of Exoskeleton

Materials.

Specimens of pond snails, edible snails, and "slugs," and other land snails, and a collection of shells of various types.

Definitions.

Gasteropoda, the name of the class to which the snail belongs. Spire, the coiled portion of the snail shell. Aperture, the opening of the shell. Lip, the edge of the shell forming the margin of the aperture. Whorl, a single coil of the spire. Suture, the depression between the whorls. Foot, the flat disk-like structure on which a snail creeps. Breathing pore, an opening in the mantle used in respiration. Lingual ribbon, the rasp or file like tongue of the snail.

Observations.
  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

To show the Effect of a Much Reduced or Rudimentary Skeleton

Materials.

Small squids, and a few large specimens for comparison and dissection.

Definitions.

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

Materials.

Specimens of as many different kinds of mollusks as possible, charts, books.

Observations.
  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

Characteristics.
  1. What are the general characteristics of mollusks?
  2. Name the principal classes and give the characteristics of each.
Morphology.
  1. What is peculiar about the structure of a clam's heart? What is its position? Contrast with the heart of a crayfish.
  2. Make cross-sectional diagrams to show the arrangement of parts in a clam: (a) in the region of the umbone; (b) in the region just in front of the posterior muscle; (c) in the region of the anterior muscle.
  3. Describe the various types of eyes found in mollusks, and their location.
  4. Describe the tongue or lingual ribbon of the snail, and its use.
  5. What is the operculum of snails? its use?
Physiology.
  1. Describe the circulation of water through the siphons and mantle cavity of a clam. How is it caused? What three uses has it?
  2. What are the principal facts about the development of fresh-water clams?
  3. Describe the circulation of blood in a clam.
  4. What various methods of locomotion are found among mollusca?
Economics.

Write a short account of the following:—

  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

Materials.

Charts, specimens, etc. Since this is partly a review exercise, your notes and drawings of invertebrates should be at hand.

Definitions.

Exoskeleton, a protective covering developed on the outside of an animal.

Questions.
  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

To show how Color may be Protective

Materials.

Specimens such as the Kny-Scheerer mimicry collections, diagrams, etc.

Definitions.

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

To show Another Method of Protection from Enemies

Materials.

Specimens, charts, etc., illustrating animal associations.

Definitions.

Animal communities, associations of many animals of the same species in communities in which there is a greater or less division of labor. Gregarious, associations where there is but little division of labor. Parasitism, an association where one animal lives at the expense of the other. The animal on which the parasite lives is called the host. If there are two hosts during the life cycle of the parasite, the second host is called an intermediate host. Symbiosis, an association where two animals live together in mutually helpful relations. Commensalism, an association where two animals live together in relations not mutually helpful but without injury to either.

Observations and questions.

Note.—To find answers to many of these questions it will be necessary to refer to the reference books in the laboratory.

  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

Materials.

Specimens, charts, and books, showing habits of invertebrates.

Definitions.

Regeneration, the power to grow new parts of the body when parts have been lost or injured. Masking, the covering of an animal by some object or organism so as to hide its identity. Nocturnal habits, the habit of hiding in the daytime and coming out at night to feed. Terrifying attitudes, the protective attitudes assumed at times by animals in order to ward off attack.

Observations and questions.
  1. How are Sabella and Serpula protected? What advantages and disadvantages in this habit? What changes in structure are associated with this tube-dwelling habit?
  2. What two protective habits has the earthworm? Name some other animals that have similar habits.
  3. Describe the protective habits of the caddis-fly larva; of the leaf-roller moth. What benefit to the hermit crab is the colony of hydractinia growing on the snail shell which it inhabits? Give other similar cases.
  4. Name as many cases of regeneration as you can.
  5. What peculiar habits has a puss-moth larva? a dragon fly? Give other examples.
Summary.
  1. Name the various protective habits.
  2. State any advantages or disadvantages you can with reference to these protective habits.

E. Defensive Structures

Another Method of Protection from Enemies

Materials.

Specimens, charts, books, etc., to illustrate the various defensive organs found among invertebrates.

Observations and questions.
  1. Describe the stinging hairs of the paramecium.
  2. Describe the action and structure of nettle cells. Where are they located in the case of hydra; of jellyfish?
  3. What defensive organs are found among the arthropods?
  4. What are stinkbugs? What peculiar organs of defense have the caterpillars of the swallowtail butterflies?
  5. Where is the sting of a hornet located? To what in a grasshopper does it correspond? Why does a hornet or bee inflict so painful a wound?
  6. What peculiar organ of defense has a squid?
  7. Find other examples of defensive structures.
Summary.
  1. What advantages have organs of defense as a method of protection?
  2. What disadvantages?

F. Thesis

To sum up the Important Points in the Study of Adaptations for Protection

Directions.

Write a connected account of what you have found out about protection of animals from their enemies, using the following outline:—

  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