ADAPTATIONS FOR PROTECTION FROM ENEMIES
#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:--
a. 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?
b. 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?
c. 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?
d. (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:--
a. The character of the clam's body,
b. the locomotion,
c. 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._
a. Dorsal margin of the clam.
b. Side view of the clam.
c. The clam with one valve removed or lifted back.
d. The clam with one valve and one mantle lobe removed.
e. The edge of a broken shell.
f. 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._
a. Drawings to show the pond snail in various positions in the
aquarium.
b. A drawing of the slug.
c. 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._
a. The squid side view.
b. 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]
[3] This and the following questions are to be answered for each
specimen. The answers may be tabulated if desired.
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._
3. What is peculiar about the structure of a clam's heart? What is
its position? Contrast with the heart of a crayfish.
4. 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.
5. Describe the various types of eyes found in mollusks, and their
location.
6. Describe the tongue or lingual ribbon of the snail, and its use.
7. What is the operculum of snails? its use?
_Physiology._
8. Describe the circulation of water through the siphons and mantle
cavity of a clam. How is it caused? What three uses has it?
9. What are the principal facts about the development of fresh-water
clams?
10. Describe the circulation of blood in a clam.
11. What various methods of locomotion are found among mollusca?
_Economics._
Write a short account of the following:--
12. Oyster culture.
13. Typhoid-fever and oysters.
14. Clams, scallops, and other edible shellfish.
15. Pearls and pearl fisheries.
16. Fresh-water clams and the button industry.
17. Sepia, Tyrian dye, etc.
18. Harmful and useful mollusks.
19. The work of U. S. Fish Commission in propagating clams.
_Natural history._
20. 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
coelenterates? What is chiefly gained?
3. Describe the community life in one of the insects in each of the
following groups:--
a. ant, honeybee, termite.
b. bumblebee, paper wasp, hornet.
c. mining bee.
d. 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--
a. its cause,
b. its threefold nature,
c. the various kinds of adaptations.
2. The various methods of protection from enemies.
a. The exoskeleton.
b. Protective coloration.
c. Animal associations.
d. Protective habits.
e. Defensive structures.