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.

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?
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?
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?
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?
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?
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?
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.
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?
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?
What other senses, if any, do you think a crayfish has? Why do you think so?
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.

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.

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.

How large is your specimen? How does it compare in size with other crayfishes in the laboratory?
Describe the shape of the body, contrasting the anterior end with the posterior, and the dorsal surface with the ventral.
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?
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?
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?
Compare the cephalo-thorax with the abdomen as to size, shape, and flexibility.
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?
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?
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.
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?
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?
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?
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?
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?
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:—

What are the varying conditions in their surroundings which crayfishes must meet? Which are most important?
What conditions must be maintained in order that crayfishes may succeed, i.e. may live and reproduce?
How does a crayfish know what are the conditions around it?
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?)
What limits the range of crayfishes, north and south? What limits it on land? What in water?
When the crayfishes of a given locality are not well adapted to it, what can they do?

Suggested drawings.

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

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

Summary.

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

3. REVIEW AND LIBRARY WORK ON CRUSTACEA

What are arthropods?
Give the classes of arthropods with an example of each.
What are the distinguishing characteristics of the class crustacea?
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?
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?
Compare the nervous system of the crayfish with that of the earthworm as regards efficiency. Upon what do you base your answer?
Name two points in which earthworms and crayfishes are alike. Name three in which they differ.
How are crayfishes caught for market? Where do those sold in Chicago usually come from? How are they shipped?
Compare the young forms of a crayfish and a crab.
Describe any five different crustacea.
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?
Discuss the process and the advantages and disadvantages of molting, as seen in the crustacea.
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?
Give the curious myth about goose barnacles.
What crustacea are parasitic? Give an account of one.
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?
Describe some of the odd means of self-protection shown among crustacea.
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?
What is the economic value of the very small crustacea?
Discuss the value to man of the various forms of crustacea.