HEN I look at a crayfish I envy it, so rich is it in organs with which to do all that it has to do. From the head to the tail, it is crowded with a large assortment of executive appendages. In this day of multiplicity of duties, if we poor human creatures only had the crayfish's capabilities, then might we hope to achieve what lies before us.
The most striking thing in the appearance of the crayfish is the great pair of nippers on each of the front legs. Wonderfully are its "thumb and finger" put together; the "thumb" is jointed so that it can move back and forth freely; and both are armed, along the inside edge, with saw teeth and with a sharp claw at the tip so that they can get a firm grip upon an object. Five segments in these great legs can be easily seen; that joining the body is small, but each successive one is wider and larger, to the great forceps at the end. The two stout segments behind the nippers give strength, and also a suppleness that enables the claws to be bent in any direction.
The legs of the pair behind the big nippers have five segments readily visible; but these legs are slender and the nippers at the end are small; the third pair of legs is armed like the second pair; but the fourth and fifth pairs lack the pincers, and end in a single claw.
But the tale of the crayfish's legs is by no means told; for between and above the great pincers is a pair of short, small legs tipped with single claws, and fringed on their inner edges. These are the maxillapeds, or jaw-feet; and behind them, but too close to be seen easily, are two more pairs of jaw-feet. As all of these jaw-feet assist at meals, the crayfish apparently always has a "three fork" dinner; and as if to provide accommodations for so many eating utensils, it has three pairs of jaws all working sidewise, one behind the other. Two of these pairs are maxillæ and one, mandibles. The mandibles are the only ones we see as we look in between the jaw-feet; they are notched along the biting edge. Connected with the maxillæ, on each side, are two pairs of threadlike flappers, that wave back and forth vigorously and have to do with setting up currents of water over the gills.
Thus we see that, in all, the crayfish has three pairs of jaw-feet, one pair of great nippers and four pairs of walking feet, two of which also have nippers and are used for digging and carrying.
When we look upon the crayfish from above, we see that the head and thorax are fastened solidly together, making what is called a cephalothorax. The cephalothorax is covered with a shell called the carapace, which is the name given also to the upper part of the turtle's shell. The suture where the head joins the thorax is quite evident. In looking at the head, the eyes first attract our attention; each is black and oval and placed on the tip of a stalk, so it can be extended or retracted or pushed in any direction, to look for danger. These eyes are like the compound eyes of insects, in that they are made up of many small eyes, set together in a honeycomb pattern.
The long antennæ are as flexible as braided whiplashes, large at the base and ending in a threadlike tip. They are composed of many segments, the basal ones being quite large. Above the antennæ on each side, is a pair of shorter ones called antennules, which come from the same basal segment; the lower one is the more slender and is usually directed forward; the upper one is stouter, curves upward, and is kept always moving, as if it were constantly on the alert for impressions. The antennæ are used for exploring far ahead or behind the creature, and are often thrust down into the mud and gravel at the bottom of the aquarium, as if probing for treasure. The antennules seem to give warning of things closer at hand. Between the antennæ and antennules is a pair of fingerlike organs, that are hinged at the outer ends and can be lifted back, if we do it carefully.
In looking down upon a crayfish, we can see six abdominal segments and the flaring tail at the end, which is really another segment greatly modified. The first segment, or that next to the cephalothorax, is narrow; the others are about equal in size, each graceful in shape, with a widened part at each side which extends down along the sides of the creature. These segments are well hinged together so that the abdomen may be completely curled beneath the cephalothorax. The plates along the sides are edged with fringe. The tail consists of five parts, one semicircular in the center, and two fan-shaped pieces at each side, and all are margined with fringe. This tail is a remarkable organ. It can be closed or extended sidewise like a fan; it can be lifted up or curled beneath.
Looking at the crayfish from below, we see on the abdomen some very beautiful featherlike organs called swimmerets. Each swimmeret consists of a basal segment with twin paddles joined to its tip, each paddle being narrow and long and fringed with hairs. The mother crayfish has four pairs of these, one pair on each of the second, third, fourth and fifth segments; her mate has an additional larger pair on the first segment. These swimmerets, when at rest, lie close to the abdomen and are directed forward and slightly inward. When in motion, they paddle with a backward, rhythmic motion, the first pair setting the stroke and the other pairs following in succession. This motion sends the body forward, and the swimmerets are chiefly used to aid the legs in forward locomotion. A crayfish, on the bottom of a pond, seems to glide about with great ease; but place it on land, and it is an awkward walker. The reason for this difference lies, I believe, in the aid given by the swimmerets when the creature is in water. Latter says: "In walking, the first three pairs of legs pull and the fourth pair pushes. Their order of movement is as follows: The first on the right and the third on the left side move together, next the third right and the first left, then the second right and fourth left, and lastly the fourth right and second left."
When the crayfish really wishes to swim, the tail is suddenly brought into use; it is thrust out backward, lays hold of the water by spreading out widely, and then doubles under with a spasmodic jerk which pulls the creature swiftly backward.
The crayfish's appearance is magically transformed when it begins to swim; it is no longer a creature of sprawling awkward legs and great clumsy nippers; now, its many legs lie side by side supinely and the great claws are limp and flow along in graceful lines after the body, all obedient to the force which sends the creature flying through the water. I cannot discover that the swimmerets help in this movement.
The mother crayfish has another use for her swimmerets; in the spring, when she is ready to lay eggs, she cleans off her paddles with her hind legs, covers them with waterproof glue, and then plasters her eggs on them in grapelike clusters of little dark globules. What a nice way to look after her family! The little ones hatch, but remain clinging to the maternal swimmerets, until they are large enough to scuttle around on the brook bottom and look out for themselves.
The breathing apparatus of the crayfish cannot be seen without dissection. All the walking legs, except the last pair, have gills attached to that portion of them which joins the body, and which lies hidden underneath the sides of the carapace or shell. The blood is forced into these gills, sends off its impurities through their thin walls and takes in the oxygen from the water, currents of which are kept steadily flowing forward.
Crayfishes haunt still pools along brooksides and river margins and the shallow ponds of our fresh waters. There they hide beneath sticks and stones, or in caves of their own making, the doors of which they guard with the big and threatening nippers, which stand ready to grapple with anybody that comes to inquire if the folks are at home. The upper surface of the crayfish's body is always so nearly the color of the brook bottom, that the eye seldom detects the creature until it moves; and if some enemy surprises one, it swims off with terrific jerks which roil all the water around and thus covers its retreat. In the winter, our brook forms hibernate in the muddy bottoms of their summer haunts. There are many species; some in our Southern States, when the dry season comes on, live in little wells which they dig deep enough to reach water. They heap up the soil which they excavate around the mouth of the well, making well-curbs of mud; these are ordinarily called "crawfish chimnies." The crayfishes find their food in the flotsam and jetsam of the pool. They seem fond of the flesh of dead fishes and are often trapped by its use as bait.
The growth of the crayfish is like that of insects; as its outer covering is a hard skeleton that will not stretch, it is shed as often as necessary; it breaks open down the middle of the back of the carapace, and the soft-bodied creature pulls itself out, even to the last one of its claws. While its new skin is yet elastic, it stretches to its utmost; but this skin also hardens after a time and is, in its turn, shed. Woe to the crayfish caught in this helpless, soft condition after molting! For it then has no way to protect itself. We sometimes find the old skin floating, perfect in every detail, and so transparent that it seems the ghost of a crayfish.
Not only is the crayfish armed in the beginning with a great number of legs, antennæ, etc., but if it happens to lose any of these organs, they will grow again. It is said that, when attacked, it can voluntarily throw off one or more of its legs. We have often found one of these creatures with one of the front claws much larger than the other; it had probably lost its big claw in a fight, and the new growth was not yet completed.
I have been greatly entertained by watching a female crayfish make her nest in my aquarium which has, for her comfort, a bottom of three inches of clean gravel. She always commences at one side by thrusting down her antennæ and nippers between the glass and stones; she seizes a pebble in each claw and pulls it up and in this way starts her excavation; but when she gets ready to carry off her load, she comes to the task with her tail tucked under her body, as a lady tucks up her skirts when she has something to do that requires freedom of movement. Then with her great nippers and the two pairs of walking feet, also armed with nippers, she loads up as much as she can carry between her great claws and her breast. She keeps her load from overflowing by holding it down with her first pair of jaw-feet, just as I have seen a schoolboy use his chin, when carrying a too large load of books; and she keeps the load from falling out by supporting it from beneath with her first pair of walking legs. Thus, she starts off with her "apron" full, walking on three pairs of feet, until she gets to the dumping place; then she suddenly lets go and at the same time her tail straightens out with a gesture which says plainly, "There!" Sometimes when she gets a very large load, she uses her second pair of walking legs to hold up the burden, and crawls off successfully, if not with ease, on two pairs of legs,—a most unnatural quadruped.
I had two crayfishes in a cage in an aquarium, and each made a nest in the gravel at opposite ends of the cage, heaping up the debris into a partition between them. I gave one an earthworm, which she promptly seized with her nippers; she then took up a good sized pebble in the nippers of her front pair of walking legs, glided over to the other nest, spitefully threw down both worm and pebble on top of her fellow prisoner, and then sped homeward. Her victim responded to the act by rising up and expressing perfectly, in his attitude and the gestures of his great claws, the most eloquent of crayfish profanity. In watching crayfishes carry pebbles, I have been astonished to see how constantly the larger pair of jaw-feet are used to help pick up and carry the loads.
Leading thought—The crayfish, or crawfish, as it is sometimes called, has one pair of legs developed into great pincers for seizing and tearing its food and for defending itself from enemies. It can live in mud or water. It belongs to the same animal group as do the insects, and it is a near cousin of the lobster.
Method—Place a crayfish in an aquarium (a battery jar or a two-quart Mason jar) in the schoolroom, keeping it in clear water until the pupils have studied its form. It will rise to explore the sides of the aquarium at first, and thus show its mouth parts, legs and swimmerets. Afterwards, place gravel and stone in the bottom of the aquarium, so that it can hide itself in a little cavity which it will make by carrying pebbles from one side. Wash the gravel well before it is put in, so that the water will be unclouded and the children can watch the process of excavation.
1. What is there peculiar about the crayfish which makes it difficult to pick it up? Examine one of these great front legs carefully and see how wonderfully it is made. How many parts are there to it? Note how each succeeding part is larger from the body to the claws. Note the tips which form the nippers or chelae, as they are called. How are they armed? How are the gripping edges formed to take hold of an object? How wide can the nippers be opened, and how is this done? Note the two segments behind the great claw and describe how they help the work of the nippers.
2. Study the pair of legs behind the great claws or chelae, and compare the two pairs, segment by segment. How do they differ except as to size? How do the nippers at the end compare with the big ones? Look at the next pair of legs behind these; are they similar? How do the two pairs of hind legs differ in shape from the two pairs in front of them?
3. Look between the great front claws and see if you can find another pair of small legs. Can you see anything more behind or above these little legs?
4. When the crayfish lifts itself up against the side of the jar, study its mouth. Can you see a pair of notched jaws that work sidewise? Can you see two or three pairs of threadlike organs that wave back and forth in and out the mouth?
5. How many legs, in all, has the crayfish? What are the short legs near the mouth used for? What are the great nippers used for? How many legs does the crayfish use when walking? In what order are they moved? Is the hind pair used for pushing? What use does it make of the pincers on the first and second pairs of walking legs?
6. Look at the crayfish from above; the head and the covering of the thorax are soldered together into one piece. When this occurs, the whole is called a cephalothorax; and the cover is called by the same name as the upper shell of the turtle, the carapace. Can you see where the head is joined to the thorax?
7. Look carefully at the eyes. Describe how they are set. Can they be pushed out or pulled in? Can they be moved in all directions? Of what advantage is this to the crayfish?
8. How many antennæ has the crayfish? Describe the long ones and tell how they are used. Do the two short ones on each side come from the same basal segment? These little ones are called the antennules. Describe the antennules of each side and tell how they differ. Can you see the little fingerlike organs which clasp above the antenna and below the antennules on each side of the head? Can these be moved?
9. Look at the crayfish from above. How many segments are there in the abdomen? Note how graceful the shape of each segment. Note that each has a fan-shaped piece down the side. Describe how the edges of the segments along the sides are margined.
10. Of how many pieces is the tail made? Make a sketch of it. How are the pieces bordered? Can the pieces shut and spread out sidewise? Is the tail hinged so it can be lifted up against the back or curled under the body?
11. Look underneath the abdomen and describe the little fringed organs called the swimmerets. How many are there?
12. How does the crayfish swim? With what does it make the stroke? Describe carefully this action of the tail. When it is swimming, does it use its swimmerets? Why do not the many legs and big nippers obstruct the progress of the crayfish, when it is swimming?
13. When does the crayfish use its swimmerets? Do they work so as to push the body backward or forward? Do you know to what use the mother crayfish puts her swimmerets?
14. Do you know how crayfishes breathe? Do you know what they eat and where they find it?
15. Where do you find crayfishes? Where do they like to hide? Do they go headfirst into their hiding place, or do they back in? Do they stand ready to defend their retreat? When you look down into the brook, are the crayfishes usually seen until they move? Why is this? Where do the crayfishes pass the winter? Did you ever see the crayfish burrows or mud chimnies?
16. If the crayfish loses one of its legs or antennas, does it grow out again? How does the crayfish grow?
17. Put a crayfish in an aquarium which has three inches of coarse gravel on the bottom, and watch it make its den. How does it loosen up a stone? With how many legs does it carry its burden of pebbles when digging its cave? How does it use its jaw-feet, its nippers, and its first and second pairs of walking legs in this work?