CHAPTER VI
WORMS
Suggestions:—Earthworms may be found in the daytime after a heavy rain, or by digging or turning over planks, logs, etc., in damp places. They may be found on the surface at night by searching with a lantern. Live specimens may be kept in the laboratory in a box packed with damp (not wet) loam and dead leaves. They may be fed on bits of fat meat, cabbage, onion, etc., dropped on the surface. When studying live worms, they should be allowed to crawl on damp paper or wood. An earthworm placed in a glass tube with rich, damp soil, may be watched from day to day.
External Features.—Is the body bilateral? Is there a dorsal and ventral surface? Can you show this by a test with a live worm? Do you know of an animal with dorsal and ventral surface, but not bilateral?
Fig. 69.—An Earthworm.
Can you make out a head? A head end? A neck? Touch the head and test whether it can be made to crawl backwards. Which end is more tapering? Is the mouth at the tip of the head end or on the upper or lower surface? How is the vent situated? Its shape? As the worm lies on a horizontal surface, is the body anywhere flattened? Are there any very distinct divisions in the body? Do you see any eyes?
Experiment to find whether the worm is sensitive (1) to touch, (2) to light, (3) to strong odors, (4) to irritating liquids. Does it show a sense of taste? The experiments should show whether it avoids or seeks a bright light, as a window; also whether any parts of the body are especially sensitive to touch, or all equally sensitive. What effect when a bright light is brought suddenly near it at night?
Is red blood visible through the skin? Can you notice any pulsations in a vessel along the back? Do all earthworms have the same number of divisions or rings? Compare the size of the rings or segments. Can it crawl faster on glass or on paper?
Fig. 70.—Mouth and Setæ.
Fig. 71.—Earthworm, mouth end above.
A magnifying glass will show on most species tiny bristle-like projections called setæ. How are the setæ arranged? (d, Fig. 70.) How many on one ring of the worm? How do they point? Does the worm feel smoother when it is pulled forward or backward between the fingers? Why? Are setæ on the lower surface? Upper surface? The sides? What is the use of the setæ? Are they useful below ground? Does the worm move at a uniform rate? What change in form occurs as the front part of the body is pushed forward? As the hinder part is pulled onward? How far does it go at each movement? At certain seasons a broad band, or ring, appears, covering several segments and making them seem enlarged (Fig. 71). This is the clitellum, or reproductive girdle. Is this girdle nearer the mouth or the tail?
Draw the exterior of an earthworm.
Dorsal and Ventral Surfaces.—The earthworm always crawls with the same surface to the ground; this is called the ventral surface, the opposite surface is the dorsal surface. This is the first animal studied to which these terms are applicable. What are the ventral and dorsal surfaces of a fish, a frog, a bird, a horse, a man?
The name “worm” is often carelessly applied to various crawling things in general. It is properly applied, however, only to segmented animals without jointed appendages. Although a caterpillar crawls, it is not a worm for several reasons. It has six jointed legs, and it is not a developed animal, but only an early stage in the life of a moth or butterfly. A “grubworm” also has jointed legs (Fig. 167). It does not remain a grub, but in the adult stage is a beetle. A worm never develops into another animal in the latter part of its life; its setæ are not jointed.
Fig. 72.—Food Tube of earthworm. (Top view.)
Fig. 73.—Food Tube and Blood Vessels of earthworm showing the ring-like hearts. (Side view.)
The Food Tube.—The earthworm has no teeth, and the food tube, as might be inferred from the form of the body, is simple and straight. On account of slight variation in size and structure, its parts are named the pharynx (muscular), gullet, crop, gizzard (muscular), and the long intestine extending through the last three fourths of its body (Fig. 72). The functions of the parts of the food tube are indicated by their names.
Fig. 74.
Circulation.—There is a large dorsal blood vessel above the food tube (Fig. 73). From the front portion of this tube arise several large tubular rings or “hearts” which are contractile and serve to keep the blood circulating. They lead to a ventral vessel below the food tube (Fig. 74). The blood is red, but the coloring matter is in the liquid, not in the blood cells.
Nervous System.—Between the ventral blood vessels is a nerve cord composed of two strands (see Fig. 75). There is a slight swelling, or ganglion, on each strand, in each segment (Fig. 75). The strands separate near the front end of the worm, and a branch goes up each side of the gullet and enters the two pear-shaped cerebral ganglia, or “brain” (Fig. 75).
Fig. 75.—Ganglia Near Mouth and part of nerve chain of earthworm.
Food.—The earthworm eats earth containing organic matter, the inorganic part passing through the vent in the form of circular casts found in the morning at the top of the earthworm’s hole. What else does it eat?
The earth worm needs no teeth, as it excretes through the mouth an alkaline fluid which softens and partly digests the food before it is eaten. When this fluid is poured out upon a green leaf, the leaf at once turns brown. The starch in the leaf is also acted upon. The snout aids in pushing the food into the mouth.
Kidneys.—Since oxidation is occurring in its tissues, and impurities are forming, there must be some way of removing impurities from the tissues. The earthworm does not possess one-pair organs like the kidneys of higher animals to serve this purpose, but it has numerous pairs of small tubular organs called nephridia which serve the purpose. Each one is simply a tube with several coils (Fig. 76). There is a pair on the floor of each segment (Fig. 76). Each nephridium has an inner open end within the body cavity, and its outer end opens by a pore on the surface between the setæ (Fig. 78). The nephridia absorb waste water from the liquid in the celom, or body cavity surrounding the food tube, and convey it to the outside.
Fig. 76.—Two pairs of Nephridia.
Respiration.—The skin of the earthworm is moist, and the blood capillaries approach so near to the surface of the body that the oxygen is constantly passing in from the air, and carbon dioxid passing out; hence it is constantly breathing through all parts of its skin. It needs no lungs nor special respiratory organs of any kind.
Fig. 77.—Sperm (sp) and egg glands (es) of earthworm.
Reproduction.—When one individual animal produces both sperm cells and egg cells, it is said to be hermaphrodite. This is true of the earthworm. The egg cell is always fertilized, however, not by the sperm cells of the same worm, but by sperm cells formed by another worm. The openings of these ova or egg glands consist of two pairs of small pores found on the ventral surface of the fourteenth and fifteenth segments in most species (see Fig. 77). There are also two pairs of small receptacles for temporarily holding the foreign sperm cells. One pair of the openings from these receptacles is found (with difficulty) in the wrinkle behind the ninth segment (Fig. 77), and the other pair behind the tenth segment. The sperm glands are in front of the ovaries (Fig. 77), but the sperm ducts are longer than the oviducts, and open behind them (Figs. 77, 78). The worms exchange sperm cells, but not egg cells. The reproductive girdle, or clitellum, already spoken of, forms the case which is to hold the eggs (see Fig. 71). When the sperm cells have been exchanged, and the ova are ready for fertilization, the worm draws itself backward from the collarlike case or clitellum so that it slips over the head. As it passes the fifteenth and sixteenth segments, it collects the ova, and as it passes the ninth and tenth segments, it collects the sperm cells previously received by touching another worm. The elastic, collar-like clitellum closes at the ends after it has slipped over the worm’s head, forming a capsule. The ova are fertilized in this capsule, and some of them hatch into worms in a few days. These devour the eggs which do not hatch. The eggs develop into complete but very small worms before the worms escape from the capsule.
Fig. 78.—Side view showing setæ, nephridia pores, and reproductive openings.
Habits.—The earthworm is omnivorous. It will eat bits of meat as well as leaves and other vegetation. It has also the advantage, when digging its hole, of eating the earth which must be excavated. Every one has noticed the fresh “casts” piled up at the holes in the morning. As the holes are partly filled by rains, the casts are most abundant after rains. The chief enemies of the earthworm are moles and birds. The worms work at night and retire so early in the morning that it takes a very early bird to catch a worm. Perhaps the nearest to an intelligent act the earthworm accomplishes is to conceal the mouth of its hole by plugging it with a pebble or bit of leaf. They hibernate, going below danger of frost in winter. In dry weather they burrow several feet deep.
The muscular coat beneath, and much thicker than the skin, consists of two layers: an outer layer runs around the body just beneath the skin, and an inner, thicker layer of fibers runs lengthwise. The worm crawls by shortening the longitudinal muscles. As the bristles (setæ) point backward, they prevent the front part of the body from slipping back, so the hinder part is drawn forward. Next, the circular muscles contract, and the bristles preventing the hind part from slipping back, the fore portion is pushed forward. Is the worm thicker when the hinder part is being pulled up or when the fore part is being thrust forward? Does the earthworm pull or push itself along, or does it do both? Occasionally it travels backward, e.g. it sometimes goes backward into its hole. Then the bristles are directed forward.
The right and left halves of the body are counterparts of each other, hence the earthworm is bilaterally symmetrical. The lungs and gills of animals must always be kept moist. The worm cannot live long in dry air, for respiration in the skin ceases when it cannot be kept moist, and the worm smothers. Long immersion in water is injurious to them, perhaps because there is far less oxygen in water than in the air.
Darwin wrote a book called “Vegetable Mold and Earthworms.” He estimated that there were fifty thousand earthworms to the acre on farm land in England, and that they bring up eighteen tons of soil in an acre each year. As the acids of the food tube act upon the mineral grains that pass through it, the earthworm renders great aid in forming soil. By burrowing it makes the soil more porous and brings up the subsoil.
Although without eyes, the worm is sensitive to light falling upon its anterior segments. When the light of a lantern suddenly strikes it at night, it crawls quickly to its burrow. Its sense of touch is so keen that it can detect a light puff of breath. Which of the foods kept in a box of damp earth disappeared first? What is indicated as to a sense of taste?
Why is the bilateral type of structure better adapted for development and higher organization than the radiate type of the starfish? The earthworm’s body is a double tube; the hydra’s body is a single tube; which plan is more advantageous, and why? Would any other color do just as well for an earthworm? Why, or why not?
The sandworm (Nereis) lives in the sand of the seashore, and swims in the sea at night (Fig. 79). It is more advanced in structure than the earthworm, as it has a distinct head (Fig. 80), eyes, two teeth, two lips, and several pairs of antennæ, and two rows of muscular projections which serve as feet. It is much used by fishermen for bait. If more easily obtained, it may be studied instead of the earthworm.
Fig. 79.—Sand Worm × ²⁄₃ (Nereis).
There are four classes in the branch Vermes: 1) the earthworms, including sandworms and leeches; 2) the roundworms, including trichina, hairworms, and vinegar eels; 3) flatworms, including tapeworm and liver fluke; 4) rotifers, which are mere specks in size.
Fig. 80.—Head of Sandworm (enlarged).
The tapeworm is a flatworm which has lost most of its organs on account of its parasitic life. Its egg is picked up by an herbivorous animal when grazing. The embryo undergoes only partial development in the body of the herbivorous animal, e.g. an ox. The next stage will not develop until the beef is eaten by a carnivorous animal, to whose food canal it attaches itself and soon develops a long chain of segments called a “tape.” Each segment absorbs fluid food through its body wall. As the segments at the older end mature, each becomes full of germs, and the segments become detached and pass out of the canal, to be dropped and perhaps picked up by an herbivorous animal and repeat the life cycle.
The trichina is more dangerous to human life than the tapeworm. It gets into the food canal in uncooked pork (bologna sausage, for example), multiplies there, migrates into the muscles, causing great pain, and encysts there, remaining until the death of the host. It is believed to get into the bodies of hogs again when they eat rats, which in turn have obtained the cysts from carcasses.
Summary of the Biological Process.—An earthworm is a living machine which does work (digging and crawling; seizing, swallowing, and digesting food; pumping blood; growing and reproducing). To do the work it must have a continual supply of energy. The energy for its work is set free by the protoplasm (in its microscopic cells) undergoing a destructive chemical change (oxidation). The waste products from the breaking down of the protoplasm must be continually removed (excretion). The broken-down protoplasm must be continually replaced if life is to continue (the income must exceed the outgo if the animal is still growing). The microscopic cells construct more protoplasm out of food and oxygen (assimilation) supplied them by the processes of nutrition (eating, digesting, breathing, circulating). This protoplasm in turn oxidizes and releases more energy to do work, and thus the cycle of life proceeds.
CHAPTER VII
CRUSTACEANS
Crawfish
Suggestions.—In regions where crawfish are not found, a live crab may be used. Locomotion and behavior may be studied by providing a tub of water, or better, a large glass jar such as a broad candy jar. For suggestions on study of internal structure, see p. 58.
Habitat.—Do you often see crawfish, or crayfish, moving about, even in water where they are known to be abundant? What does your answer suggest as to the time when they are probably most active?
Why do you never see one building its chimney, even where crawfish holes are abundant? Is the chimney always of the same color as the surface soil? Are the crawfish holes only of use for protection? In what kind of spots are crawfish holes always dug? Why? What becomes of crawfish when the pond or creek dries up? How deep are the holes? How large are the lumps of mud of which the chimney is built? How does it get them out of the hole? Why is the mud built into a chimney instead of thrown away? (What would happen to a well with its mouth no higher than the ground?) Why are crawfish scarce in rocky regions, as New England?
How does the color of the crawfish compare with its surroundings? Is its color suited to life in clear or muddy water? Define protective coloration.
Habits.—Does the crawfish walk better in water or out of it? Why? Does it use the legs with the large claws to assist in walking? Do the swimmerets (under the abdomen) move fast or slow? (Observe it from below in a large jar of clear water.) What propels it backward? Forward? Does the crawfish move at a more uniform rate when swimming backward or forward? Why? In which way can it swim more rapidly? Do the big legs with claws offer more resistance to the water while it is swimming backward or forward? How does it hold the tail after the stroke, while it is darting backward through the water? Hold a crawfish with its tail submerged and its head up. Can the tail strike the water with much force? Allow it to grasp a pencil: can it sustain its own weight by its grip?
Feeding.—Offer several kinds of food to a crawfish that has not been alarmed or teased. Does it prefer bread, meat, or vegetables? How does it get the food to its mouth? Does it eat rapidly or slowly? Does it tear the food with the big pincers? Can it gnaw with the small appendages near the mouth?
Breathing.—Does the crawfish breathe with gills or lungs? Place a few drops of ink near the base of the hind legs of a crawfish resting quietly in shallow water. Where is the ink drawn in? Where does it come out? To explain the cause and purpose of this motion, place a crawfish in a large glass jar containing water, and see the vibratory motion of the parts under the front portion of the body. There is a gill paddle, or gill bailer, under the shell on each side of the body that moves at the same rate.
Senses.—Crawfish are best caught with a piece of meat or beef’s liver tied to a string. Do they always lose hold as soon as they are lifted above the water? What do you conclude as to the alertness of their senses? Does the covering of its body suggest the possession of a delicate or dull sense of touch?
Of what motions are the eyes capable? Touch one of the eyes. The result? Can a crawfish see in all directions? To test this, place a crawfish on a table and try whether you can move to a place where you can see the crawfish without seeing its eyes. What are the advantages and disadvantages of having the eyes on stalks?
Fig. 81.—Crawfish (dorsal surface).
Fig. 82.
Touch the body and the several appendages of the crawfish. Where does it seem most sensitive to touch? Which can reach farther, the antennæ or the big claws? Why are short feelers needed as well as long ones?
Make a loud and sudden noise without jarring the crawfish. Is it affected by sound?
External Anatomy (Figs. 81, 82, 83, 84).—Is the body of the crawfish rounded out (convex) everywhere, or is any part of its surface either flat or rounded in (concave)? What color has the crawfish? Is this color of any use to the crawfish?
Fig. 83.—Lateral view of Crawfish.
Fig. 84.—Fourth Abdominal Segment of Crawfish with swimmeret.
Make out the two distinct regions or divisions of the body (Fig. 81). The anterior (front) region is called the head-chest or cephalothorax, and the posterior (rear) region is called the tail. Which region is larger? Why? Which is flexible? Why?
Is the covering of the body hard or soft? What is the advantage of such a covering? What are its disadvantages? How is the covering modified at the joints to permit motion?
Tail.—How many joints, or segments, on the tail? (Figs. 81, 83.) Does the hard covering of each segment slip under or over the segment behind it when the abdomen is straight? Does this lessen friction while swimming forward?
Is there a pair of swimmerets to each segment of the abdomen? (Figs. 82, 86.) Notice that each swimmeret has a main stalk (protopod), an outer branch (exopod), and an inner branch (endopod) (Fig. 84). Are the stalk and the branches each in one piece or jointed? The middle part of the tail fin is called the telson. By finding the position of the vent, decide whether the food tube goes into the telson (Fig. 82). Should it be called an abdominal segment? Are the side pieces of the tail fin attached to the telson or to the sixth segment? Do these side pieces correspond to swimmerets? Do they likewise have the Y-shaped structure? (Fig. 86.)
If the swimmerets on the first abdominal segment are large, the specimen is a male. If they are small, it is a female. Which sex is shown in Fig. 82? Fig. 86?
Carapace.—The covering of the head chest (cephalothorax) is called the carapace. Has it free edges? The gills are on the sides of the body and are covered by the carapace (Fig. 87). The projection in front is called the rostrum, meaning beak. Does the rostrum project beyond the eyes? There is a transverse groove across the carapace which may be said to divide the head from the abdomen. Where does this groove end at the sides?
Fig. 85.—1, mandible; 2, 3, maxillæ; 4, 5, 6, maxillipeds.
Legs.—How many legs has the crawfish? How many are provided with large claws? Small claws? Is the outer claw hinged in each of the large grasping pincers? The inner claw?
Fig. 86.—Crawfish (ventral surface).
Appendages for Taking Food.—If possible to watch a living crawfish eating, notice whether it places the food directly into the mouth with the large claws. Bend the large claws under and see if they will reach the mouth.
Attached just in front of the legs the crawfish has three pairs of finger-like appendages, called foot jaws (maxillipeds), with which it passes the food from the large pincers to its mouth (Figs. 85, 86). They are in form and use more like fingers than feet. In front of the foot jaws are two pairs of thin jaws (maxillæ) and in front of the thin jaws are a pair of stout jaws (mandibles) (Fig. 85). Do the jaws move sidewise or up and down? Which of the jaws has a jointed finger (palp) attached to it? Do all of the appendages for taking food have both exopod and endopod branches on a basal stalk or protopod? Which of the appendages have a scalloped edge? How would you know from looking at the crawfish that it is not merely a scavenger? Why are there no pincers on the hind feet?
Fig. 87.—Gill cover removed and gills exposed. Mp, gill bailer.
Sense Organs.—Find the antennæ, or long feelers (Figs. 82, 90). Are the antennæ attached above or below the eyes? (Fig. 87.)
Fig. 88.—Lengthwise Section of Male Crawfish.
c, heart; Ac, artery to head; Aa, artery to abdomen; Km, stomach; D, intestine; L, liver; T, spermary; Go, opening of sperm duct; G, brain; N, nerve chain.
Find the pair of antennules, or small feelers. Are their divisions like or unlike each other? Compare the length of the antennules and the antennæ. Compare the flexibility of the antennæ with that of the other appendages.
Observe the position of the eyes (Figs. 81, 88). How long are the eyestalks? Is the stalk flexible or stiff? Touch the eye. Where is the joint which enables the stalk to move? Is the outer covering of the eye hard or soft? A mounted preparation of the transparent covering (cornea) of the eye, seen with lower power of microscope, reveals that the cornea is made up of many divisions, called facets. Each facet is the front of a very small eye, hundreds of which make up the whole eye, which is therefore called a compound eye. The elongated openings to the ear sacs are located each on the upper side of the base of a small feeler just below the eye.
Respiratory System.—The respiratory organs are gills located on each side of the thorax in a space between the carapace and body (Fig. 87). The gills are white, curved, and feathery. Is the front gill the largest or the smallest? The gills overlap each other; which is the outermost gill? On the second maxilla is a thin, doubly curved plate called a gill bailer (Fig. 85). The second maxilla is so placed that the gill bailer comes at the front end of the gill chamber. The bailer paddles continually, bringing the water forward out of the gill. The gills are attached below at the base of the legs. Are the gills thick or thin? How far upward do they go? Does the backward motion in swimming aid or hinder the passage of the water through the gills? Does a crawfish, when at rest on the bottom of a stream, have its head up or down stream? Why?
Openings.—The slitlike vent is on the under side of the telson (Figs. 82, 88). The mouth is on the under side of the thorax behind the mandibles. At the base of the long antennæ are the openings from the green glands, two glands in the head which serve as kidneys (Fig. 89). The openings of the reproductive organs are on the third pair of legs in the female, and the fifth pair of legs in the male (Fig. 88). The eggs are carried on the swimmerets.
Fig. 89.—Level lengthwise section showing
h, heart.
d, green gland.
le, liver.
kie, gills.
kh, gill cavity.
ma, stomach.
(After Huxley.)
Internal Structure.—Suggestions.If studied by dissection, it will be necessary to have several crawfish for each pupil, one for gaining general knowledge, and others for studying the systems in detail. Specimens should have lain in alcohol for several days.
Fig. 90.—Section of Crawfish showing stomach s, liver li, and vent a.
The Food Tube.—Is the stomach in the head portion of the cephalothorax or in the thoracic portion? (Figs. 88, 89). Is the stomach large or small? What is its general shape? Does the gullet lead upward or backward? Is it long or short? (Fig. 88.) The mid tube, which is the next portion of the food tube, is smaller than the stomach. On each side of it are openings from the bile ducts which bring the secretion from the digestive gland, sometimes called the liver. Does this gland extend the whole length of the thorax? Is it near the floor or the top of the cavity? The third and last portion of the food tube is the intestine. It extends from the thorax to the vent. Is it large or small? Straight or curved? The powerful flexor muscles of the tail lie in the abdomen below the intestines. Compare the size of these muscles with the extensor muscle above the intestine (Fig. 90). Why this difference? Does the food tube extend into the telson? Locate the vent (Fig. 90).
The Circulation.—The blood is a liquid containing white corpuscles. It lacks red corpuscles and is colorless. The heart is in the upper part of the thorax. It is surrounded by a large, thin bag, and thus it is in a chamber (called the pericardial sinus). The blood from the pulmonary veins enters this sinus before it enters the heart. The origin of this pericardial sinus by the fusing of veins is shown in Fig. 130. Does one artery, or do several arteries, leave the heart? There is a larger dorsal artery lying on the intestine and passing back to the telson; there are three arteries passing forward close to the dorsal surface (Figs. 89, 91). One large artery (the sternal) passes directly downward (Figs. 88, 91), and sends a branch forward and another backward near the ventral surface. The openings into the heart from the sinus have valvular lips which prevent a backward flow of blood into the sinus. Hence, when the heart contracts, the blood is sent out into the several arteries. The arteries take a supply of fresh blood to the eyes, stomach, muscles, liver, and the various organs. After it has given oxygen to the several organs and taken up carbon dioxid, it returns by veins to pass through the gills on each side, where it gives out the useless gas and takes up oxygen from the water. It is then led upward by veins into the pericardial sinus again.
A double nerve chain of ganglia supplies nerve force to the various nerves (Fig. 92). This main nerve chain lies along the ventral surface below the food tube (Fig. 90), except one pair of ganglia which lie above the esophagus or gullet (Fig. 88), and are called the supra-esophageal ganglia, or brain.
Fig. 91.—Showing heart and main blood vessels.
Fig. 92.
Crustacea.—Because of the limy crust which covers the crawfish and its kindred, they are placed in the class called Crustacea.
Fig. 93.—Crab from below.
Fig. 94.—Hermit Crab, using shell of sea snail for a house.
Decapods.—All crustacea which have ten feet belong in the order called decap′oda (ten-footed). This order includes the crabs, lobsters, shrimp, etc. The crabs and lobsters are of considerable importance because of use as food. Small boys sometimes catch crawfish, and in some instances are known to cook and eat them for amusement, the only part cooked being the muscular tail. The crab’s tail is small and flat and held under the body (Fig. 93).
Fig. 95.—Development of a Crab.
a, nauplius just after hatching; b, c, d, zoëa; e, megalops; f, adult.
Question: Which stage is most like a crayfish? Compare with metamorphoses of insects.
Since the limy covering to serve the purpose of protection is not soft enough to be alive and growing, it is evident that the crustacea are hampered in their growth by their crusty covering. During the first year the crawfish sheds its covering, or molts three times, and once each year thereafter. It grows very fast for a few days just after molting, while the covering is soft and extensible. Since it is at the mercy of birds, fish, and other enemies while in this soft and defenseless condition, it stays hidden until the covering hardens. Hence it cannot eat much, but probably by the absorption of water the tissues grow; that is, enlarge. In the intervening periods, when growth is impossible, it develops; that is, the tissues and organs change in structure and become stronger. “Soft-shelled crab” is a popular dish, but there is no species by that name, this being only a crab just after molting which has been found by fishermen in spite of its hiding.
General Questions.—How do crawfish choose their food? How long can they live out of water? Why do their gills remain moist out of water longer than a fish? How do they breathe out of water? Are they courageous or cowardly animals? When they lose appendages when fighting or molting, they are readily reproduced, but the part molts several times in regaining its size. Have you seen crawfish with one claw smaller than the other? Explain.
Compare the crawfish and crab (Figs. 81, 93, and 95) in the following particulars: shape, body, eyes, legs, abdomen, habitat, movement.
KEY TO THE FOUR CLASSES IN BRANCH ARTHROPODS
| 1. | Insects | 3 body divisions, 6 legs |
| 2. | Arachnids | 2 body divisions, 8 legs |
| 3. | Myriapods | many body divisions, many legs |
| 4. | Crustaceans | gill breathers, skeleton (external) limy |
By the aid of the key and of figures 96-105, classify the following Arthropods: tick, thousand-leg centipede, king crab, pill bug, spider, scorpion, beetle.
| Fig. 96.—Pill Bug. | Fig. 97.—Lady Beetle. | Fig. 98.—Scorpion. | |
| Fig. 101.—One Segment of Centipede with one pair of legs. | Fig. 99.—Tick before and after feeding. | ||
| Fig. 102.—One Segment of Thousand Legs with two pairs of legs. | Fig. 103.—Thousand Legs. | ||
| Fig. 100.—Centipede. | Fig. 104.—A Spider. | Fig. 105.—King Crab. | |
Illustrated Study. Classification of Arthropods. Key on p. 61.