CRUSTACEANS
CRAWFISH
SUGGESTIONS.—In regions where crayfish are not found, a live crab may
be used. Locomotion and behaviour 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 crayfish holes
are abundant? Is the chimney always of the same colour as the surface
soil? Are the crayfish holes only of use for protection? In what kind of
spots are crayfish always dug; Why? What becomes of crayfish when the
pond or the 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 crayfish scarce in rocky regions?
How does the colour of the crayfish compare with its surroundings? Is
its colour suited to live in clear or muddy water? Define protective
colouration.
=Habits.=—Does the crayfish 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 crayfish 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 crayfish 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 crayfish 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 crayfish breathe with gills or lungs? Place a few
drops of ink near the base of the hind legs of a crayfish resting
quietly in shallow water. Where is the ink drawn in? Where does it come
out? To explain the cause and the purpose of this motion, place a
crayfish in a large glass jar containing water, and see the vibratory
motion of the parts under the front portion of the body. There is under
the shell on each side of the body a gill paddle, or gill bailer, that
moves at the same rate.
=Senses.=—Crayfish 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 their bodies suggest the possession of a
delicate or a dull sense of touch?
Of what motions are the _eyes_ capable? Touch one of the eyes. The
result? Can a crayfish see in all directions? To test this, place a
crayfish on a table and try whether you can move to a place where you
can see the crayfish 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 crayfish. 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 crayfish. Is it
affected by _sound_?
=External Anatomy= (Figs. 81, 82, 83, 84).—Is the body of the crayfish
rounded out (convex) everywhere, or is any part of its surface either
flat or rounded in (concave)? What _colour_ has the crayfish? Is this
colour of any use to the crawfish?
[Illustration:
FIG. 83.—LATERAL VIEW OF CRAWFISH.
]
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?
[Illustration:
FIG. 84.—FOURTH ABDOMINAL SEGMENT OF CRAWFISH with swimmeret.
]
=Tail.=—How many joints, or segments, of the tail? (Figs. 81, 83.) Does
the hard covering of each segment slip under or over the segment behind
it when the tail is straight? Does this lessen friction while swimming
forward?
Is there a pair of _swimmerets_ to each segment of the tail? (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.)
[Illustration:
FIG. 85.—1, mandible; 2,3, maxillæ; 4,5,6, maxillipeds.
]
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?
[Illustration:
FIG. 86.—CRAYFISH (ventral surface).
]
=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?
=Legs.=—How many legs has the crayfish? How many are provided with large
claws? Small claws? Is the outer claw hinged in each of the large
grasping pincers? The inner claw?
=Appendages for Taking Food.=—If possible to watch a living crayfish
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 crayfish 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 in 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 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
crayfish 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 the 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
crayfish, 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 crayfish 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 CRAYFISH 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).
[Illustration:
FIG. 91.—Showing heart and main blood vessels.
]
=The Circulation.=—The blood is a liquid containing _white
corpuscles_. It lacks red corpuscles and is colourless. 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 dioxide, 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.
[Illustration:
FIG. 92.
]
The central nervous system consists of a _double chain_ of ganglia
(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 œsophagus or gullet (Fig. 88), and are called the supra-œsophageal
ganglia, or brain.
=Crustacea.=—The crayfish and its kindred 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 the lobsters are of considerable importance because
of use as food. Small boys sometimes catch crayfish, 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 crayfish sheds its covering, or =moults= three times, and once each
year thereafter. It grows very fast for a few days just after moulting,
while the covering is soft and extensible. Since it is at the mercy of
birds, fish, and other enemies while in this soft and defenceless
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 moulting which has been
found by fishermen in spite of its hiding.
=General Questions.=—How do crayfish 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 in fighting
or moulting these are readily reproduced, but an organ moults several
times in regaining its size. Have you seen crayfish with one claw
smaller than the other? Compare the crayfish 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.