2. How many cells are necessary to do this work?
3. When this work is divided among a number of cells, what is the effect upon the quant.i.ty and quality of work accomplished?
4. When this work is divided among a number of cells, how does the structure of the cells show it? How does the arrangement of the cells also show it? Give examples.
5. The technical expression for this specialization of cells, giving them different functions, is "division of labor." Formulate a clear definition for this expression, giving an example to ill.u.s.trate it.
6. Is division of labor a good thing for an animal body, or is it not? Give reasons for your opinion, with examples for ill.u.s.tration.
CHAPTER IV
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 sh.e.l.l-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 cla.s.sed as appendages by all students.)
_Antennules_, the small feelers.
_Antennae_, the large feelers.
_Mandibles_, the jaws, one on each side of the mouth.
_Maxillae_, the two pairs of small mouth parts just back of the mandibles.
_Maxillipeds_, the three pairs of appendages between the maxillae and the large claws.
_Chelipeds_, the large claws or pinchers.
_Walking legs_, the four pairs of appendages back of the chelipeds.