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?

[Illustration: FIG. 81.--CRAWFISH (dorsal surface).]

[Illustration: 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?

[Illustration: FIG. 83.--LATERAL VIEW OF CRAWFISH.]

[Illustration: 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?

[Illustration: 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?

[Illustration: 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?

[Illustration: 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.)

[Illustration: 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.

[Illustration: 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.

[Illustration: 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.

[Illustration: FIG. 91.--Showing heart and main blood vessels.]

[Illustration: FIG. 92.]

=Crustacea.=--Because of the limy crust which covers the crawfish and its kindred, they are placed in the class called _Crustacea_.

[Illustration: FIG. 93.--CRAB FROM BELOW.]

[Illustration: 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).

[Illustration: 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.

[Illustration: FIG. 96.--PILL BUG.]

[Illustration: FIG. 97.--LADY BEETLE.]

[Illustration: FIG. 98.--SCORPION.]

[Illustration: FIG. 99.--TICK before and after feeding.]

[Illustration: FIG. 100.--CENTIPEDE.]

[Illustration: FIG. 101.--ONE SEGMENT OF CENTIPEDE with one pair of legs.]

[Illustration: FIG. 102.--ONE SEGMENT OF THOUSAND LEGS with two pairs of legs.]

[Illustration: FIG. 103.--THOUSAND LEGS.]

[Illustration: FIG. 104.--A SPIDER.]

[Illustration: FIG. 105.--KING CRAB.]

=Illustrated Study.= CLASSIFICATION OF ARTHROPODS. Key on p. 61.