ADAPTATION TO SURROUNDINGS
#A STUDY OF CRUSTACEA#
#To Show the General Adaptation of an Invertebrate to its
Surroundings#
1. A STUDY OF CRAYFISHES
_Materials._
Crayfishes, living and preserved. Some of the living crayfishes
should be established in conditions as natural as possible _i.e._ in
an inch or so of fresh water, with rocks, weeds, etc., and left
undisturbed. Small crayfishes are desirable to show locomotion in
water.
Living Crayfishes
_Directions and observations._
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
_Definitions._
_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
To summarize your study, write a connected account of the relations
of crayfishes to their environments, under the following topics:--
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):--
a. 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?
b. 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?
c. What ranges of temperature can crayfishes endure? What temperature
is best? How do they avoid fatal extremes?
d. What are the enemies of crayfishes? What protection against these
have they?
e. 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._
a. The whole animal, dorsal surface, preferably without appendages.
b. One of each pair of appendages, except where they duplicate.
c. The tail-fin. Label the sixth swimmerets, the sixth and seventh
somites.
d. The gill chamber, with gills in position. Show circulation of
water by arrows.
e. 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.