BRANCH ARTHROPODA: CRUSTACEANS, CENTIPEDS,
INSECTS, AND SPIDERS
The great branch Arthropoda includes a host of familiar animals. It
contains more species than any other branch of the animal kingdom. To it
belong the crayfishes, shrimps, crabs, lobsters, water-fleas, and other
animals which compose the class Crustacea; the centipeds and
thousand-legged worms which compose the class Myriapoda; the true or
six-footed insects forming the class Insecta, which includes nearly
two-thirds of all the known species of animals; and the scorpions,
mites, ticks, and spiders which constitute the class Arachnida. There is
also a fifth class in the branch Arthropoda which includes a few species
of animals unfamiliar to us but of great interest to zoologists.
All these varied kinds of animals have a body on the annulate or
segmented type-plan, like that shown by most worms, but they differ from
the worms in possessing jointed appendages, used for locomotion or food
taking. There is typically or racially one pair of these jointed or
segmented appendages on each segment of the body, but in all of the
Arthropoda some of the segments have lost their appendages. The body is
covered by a firm cuticle or outer body-wall called the exoskeleton.
This exoskeleton serves not only to enclose and protect the soft parts
of the body but also for the attachment of the body muscles. It may be
flexible as in the sutures between the body-segments in most insects, or
hard and rigid as in the sclerites of the segments. The firmness is due
primarily, and in the insects usually solely, to a deposit in the
cuticle of _chitin_, a substance probably secreted by the underlying
cells of the true skin, or it may be due chiefly, as in the crabs, to a
calcareous deposit. In such cases it becomes a veritable armor. The
internal organs of the Arthropods show a more or less obvious
segmentation corresponding with the segmentation of the body-wall. The
alimentary canal runs longitudinally through the center of the body from
mouth to anal opening. The nervous system consists of a brain lying
above the œsophagus and a double nerve-chain running backward from
beneath the œsophagus, along the median line of the ventral wall, to the
posterior extremity of the body. This ventral nerve-chain consists of a
pair of longitudinal commissures or cords and a series of pairs of
ganglia, arranged segmentally. The two ganglia of each pair are fused
more or less nearly completely to form a single ganglion, and the
nerve-cords are partially fused, or at least lie close together. In
addition there is a smaller sympathetic system composed of a few small
ganglia and certain nerves running from them to the viscera, this system
being connected with the main or central nervous system. In this group
the organs of special sense reach for the first time a high stage of
development. Compound eyes are peculiar to Arthropoda. The heart lies
above the alimentary canal. Respiration is carried on by gills in the
aquatic forms, and by a remarkable system of air-tubes or tracheæ in the
land forms (insects). The sexes are usually distinct, and reproduction
is almost universally sexual. Most of the species lay eggs.
The Arthropods are animals of a high degree of organization. The
extremely diverse life-habits of the various kinds among them have
led to much modification and to great specialization of structure. The
course of development, too, is made very complicated by the elaborate
metamorphosis undergone by many of the members of the branch.
We shall study the Arthropoda by getting acquainted with a few examples
of each class and thus learning the special class characteristics.
CLASS CRUSTACEA: CRAYFISHES, CRABS, LOBSTERS,
ETC.
THE CRAYFISH (_Cambarus_ sp.)
=Structure.=--The structure of the crayfish has been already studied
(see Chapter IV and figs. 3 and 4).
=Life-history and habits.=--Crayfish frequent fresh-water lakes, rivers,
and springs in most parts of the United States. Many of them perish
whenever the small prairie ponds dry up. But some burrow into the earth
when the dry season comes. There may be noticed in meadows where water
stands for certain seasons of the year many scattered holes with slight
elevations of mud about them. These are mostly the burrows of crayfish.
During the dry season the crayfish digs down until it reaches water, or
at least a damp place, where it rests until wet weather brings it to the
surface once more. One of these burrows, followed in digging a mining
shaft, extended vertically down to a distance of twenty-six feet, where
the crayfish was found tucked snugly away.
The eggs are carried by the female on her abdominal appendages. Previous
to the laying of the eggs the female rubs off all foreign matter from
the appendages, thus preparing them for the reception of the eggs. This
cleaning is done with the fifth pair of legs. When the eggs are ready
to be laid, which is during the last of March or in April in the Central
States, a sticky secretion passes out of the openings at the base of the
walking legs and smears the pleopods of the abdomen. The eggs as they
pass out are fertilized and caught on the pleopods, where they remain
attached in clusters. After some weeks the young crayfishes issue from
the eggs. In general appearance they are not very unlike the adults.
They grow very rapidly at this stage. As the animal is enclosed in a
hard shell, growth can only take place during the period just following
the molt, for the crayfish casts its skin periodically, and it is while
the new shell is forming that the animal does its growing. The crayfish
when it molts casts not only the exoskeleton, but also the lining of
part of the alimentary canal. After the females have hatched their young
many die in the shallow pools, in which places the dried-up skeletons
are noticeable during the summer months.
OTHER CRUSTACEANS.
Most of the crustaceans live in water, a few being found in damp soil
or in other moist places. Some are fresh-water animals and some
marine. They vary in size from the tiny water-fleas, a millimeter
long, to crabs two feet across the shell or sixteen feet from tip to
tip of legs. They present great differences in form and general
appearance of body, being adapted for various conditions of life. Some
crustaceans live as parasites on other animals, in some cases on other
crustaceans. Such parasitic species have the body much modified and
are hardly to be recognized as members of the class.
=Body form and structure.=--In structural character and body
organization the Crustaceans show, of course, the general
characteristics already attributed to the Arthropoda, the branch to
which they belong. The characteristics which distinguish them from
other Arthropods are the possession of gills for respiration (some
insects have gills, but of a very different kind as will be seen
later), and the bi-ramose condition of the body appendages, each
appendage (excepting the antennules) consisting of a single basal
segment from which arise two branches made up of one or more segments.
Of the form of the crustacean body few generalizations can be made.
"There is no [other] class in the animal kingdom which presents so
wide a range of organization as the Crustacea, or in which the
deviations in structure from the 'type form' are so striking and so
interesting from their obvious adaptation to the mode of life." For
this reason no attempt will be made to discuss in general terms the
form of the crustacean body, but brief accounts will be given of a few
of the more familiar kinds of Crustacea which will serve to illustrate
this remarkable diversity of body form.
Similarly impossible is it also to give a general account of the
development of the crustaceans. The sexes are distinct in most
Crustacea, and there is often great difference in form between the
male and female. A certain amount of metamorphosis takes place in the
development of all crustaceans; that is, the young when hatched from
the egg differs, often decidedly, in appearance and structure from the
parent, and in the course of its post-embryonic development undergoes
more or less striking change or metamorphosis. This metamorphosis is
often very marked.
=Water-fleas (Cyclops).=--TECHNICAL NOTE.--The water-fleas are
common in the water of ponds or of slow streams; they may often be
found in the school aquarium. They are, though small (about 1 mm.
long), readily seen with the unaided eye; they are white, rather
elongate, and have a rapid jerky movement. Examine specimens alive
in water in a watch glass. Note the "split pear" shape, broadest
near the front, tapering posteriorly, flat beneath, convex above;
note the forked stylets at tip of abdomen; also the two pairs of
antennæ, the single median eye, the mandibles, two pairs of
maxillæ, and five pairs of legs (last pair very small). There are
no gills. Some of the specimens, females, may have attached to the
first abdominal segment on either side an egg sac. Make drawings
showing all these structural details. Watch the _Cyclops_
capturing and feeding on _Paramœcium_ or other small animals.
[Illustration: FIG. 35.--A water-flea, _Cyclops_ sp. Female with
egg-masses. (From living specimen.)]
The water-fleas (_Cyclops_) (fig. 35) are among the smallest of the
Crustacea. They are extremely abundant, having great power of
multiplication. "An old _Cyclops_ may produce forty or fifty eggs at
once, and may give birth to eight or ten broods of children living
five to six months. As the young begin to reproduce at an early age,
the rate of multiplication is astonishing. The descendants of one
_Cyclops_ may number in one year nearly 4,500,000,000, or more than
three times the total population of the earth, provided that all the
young reach maturity and produce the full number of offspring." The
_Cyclops_ feed on smaller aquatic animals such as Protozoa, Rotifera,
etc. They in turn serve as food for fishes; and because of their
immense numbers and occurrence in all except the swiftest fresh waters
"they form the main food of most of our fresh-water fishes while
young." Many aquatic insect larvæ feed almost exclusively on them.
Related to the _Cyclops_ are a host of other kinds of minute
Crustaceans. Among these the so-called fish-lice are specially
interesting because of their parasitic habits and greatly modified and
degenerate structure. There are many kinds of these parasitic
crustaceans infesting fishes, whales, molluscs, and worms. "As on land
almost every species of bird or mammal has its own parasitic insects, so
in the water almost every species of fish or larger invertebrate has its
parasitic crustaceans." Some of the most common of these parasites
attach themselves to the gills of fishes. Here they cling, sucking the
blood or animal juices from the host. In form of body they do not at all
resemble other Crustaceans, but are strangely misshapen. They are often
worm-like, or sac-like, without legs or other locomotory appendages. As
with other parasites (see Chapter XXX) an inactive dependent life
results in the atrophy and loss by degeneration of the body-parts
concerned with locomotion and orientation.
=Wood lice (Isopoda).=--TECHNICAL NOTE.--Specimens of wood lice,
pill bugs, or damp bugs, as they are variously called, may be
readily found in concealed moist places, as under stones or boards
on damp soil. They are often common in houses, near drains or in
dark, damp places. Examine some live wood lice, and some dead
specimens (killed by chloroform or in an insect-killing bottle).
Note the division of the body into the head, thorax, and abdomen;
find the eyes, the antennæ and the mouthparts (mandibles and
maxillæ are usually pressed closely together). All the locomotory
appendages are adapted for walking or running, not swimming. Note
the number of pairs of legs; the structure of a leg; find gills
and gill-covers. Some females may be found with eggs on the under
side of the thorax near the bases of the legs, the eggs being
covered by thin membranous plates. Make drawings showing the
general form and character of body and details of legs, gills,
etc. Compare with the crayfish and _Cyclops_.
The wood-lice (fig. 36) are among the few Crustacea which have a
wholly terrestrial life. They run about quickly and feed chiefly on
decaying vegetable matter. They are night scavengers. They have the
body oval and convex above, rather purplish or grayish brown, and
smooth. Although they do not live in the water they breathe partly at
least by means of gills (though they may breathe partly through the
skin). It is therefore necessary for them to live in a damp atmosphere
so that the gill membranes may be kept damp. If not kept moist they
could not serve as osmotic membranes.
[Illustration: FIG. 36.--A damp bug, Isopod, species not determined.
(From specimen.)]
=Lobsters, Shrimps and Crabs (Decapoda).=--TECHNICAL
NOTE.--Teachers living near the sea-shore can get specimens of
live and dead lobsters, shrimps, and crabs in the markets. Schools
in the interior should have a few preserved specimens for
examination. These specimens should be compared with the crayfish;
although differences in shape of body are evident, the character
and arrangement of body parts will be found to be very similar.
The largest and most familiar Crustaceans, as the crayfishes,
lobsters, shrimps, prawns and crabs, all belong to the order Decapoda,
or ten-legged Crustacea. The members of this order have, including the
large claws, ten walking feet; they all have eyes on movable stalks,
and the front portion of the body is covered by a horny fold of the
body-wall called the carapace.
The lobsters are large ocean-inhabiting crustaceans which are very like
the fresh-water crayfish in all structural characters. They live on the
rocky or sandy ocean-bottom at shallow depths. They feed largely on
decaying animal matter. They are caught in great numbers in so-called
"lobster pots," a kind of wooden trap baited with refuse. "The number
thus taken upon the shores of New England and Canada amounts to between
twenty and thirty million annually." Live lobsters are brownish or
greenish with bluish mottling; they turn red when boiled. A single
female will lay several thousand eggs. The eggs are greenish and are
carried about by the mother until the young hatch. The young are
free-swimming larvæ, until they reach a length of half an inch.
The shrimps and prawns are mostly marine, though some species live in
fresh water. They are, like the lobsters, used for food. Some of the
species are gregarious in habit, occurring in great "schools" of
individuals. Like the lobsters they crawl about on the sea-bottom
feeding on decaying animal matter. Shrimps are very abundant near San
Francisco, where extensive "shrimp fishing" is done by the Chinese.
[Illustration: FIG. 37.--Some crabs and barnacles of the Pacific
coast; the short sessile acorn barnacles in the upper left-hand corner
belong to the genus _Balanus_; the stalked barnacles in the upper
right-hand corner are of the species _Pollicipes polymenus_; the
largest crab (upper left-hand) is _Brachynotus nudus_; the one in
left-hand lower corner is a young rock-crab, _Cancer productus_; the
crab in the sea-weed at the right is a kelp-crab, _Epialtus
productus_, while the two in snail-shells in lower corner are
hermit-crabs, _Pagurus samuelis_. (From living specimens in a
tide-pool on the Bay of Monterey, California.)]
The crabs (fig. 37) differ from the lobsters and crayfishes and
shrimps in having the body short and broad, instead of elongate. This
is due to the special widening of the carapace and the marked
shortening of the abdomen. The abdomen, moreover, is permanently bent
underneath the body, so that but little of it is visible from the
dorsal aspect. The number of abdominal legs or appendages is reduced.
When the tide is out the rocks and tide-pools of the ocean shore are
alive with crabs. They "scuttle" about noisily over the rocks,
withdrawing into crevices or sinking to the bottom of the pools when
disturbed. They move as readily backward or sidewise, "crab-fashion,"
as forward. They are of various colors and markings, often so
patterned as to harmonize very perfectly with the general color and
appearance of the rocks and sea-weeds among which they live. The
spider-crabs are especially strange-looking creatures with unusually
long and slender legs and a comparatively small body-trunk. They
include the _Macrocheira_ of Japan, the largest of the crustaceans.
Specimens of this crab are known measuring twelve to sixteen feet from
tip to tip of extended legs; the carapace is only as many inches in
width or length. The soft-shelled crab is a species common along our
Atlantic coast. It is "soft-shelled" only at the time of molting, and
has to be caught in the few days intervening between the shedding of
the old hard shell and the hardening of the new body-wall. The little
oyster-crabs (_Pinnotheres_) which live with the live oyster in the
cavity enclosed by the oyster shell are well-known and interesting
crabs. They are not parasites preying on the body of the oyster, but
are simply messmates feeding on particles of food brought into the
shell by the currents of water created by the oysters.
Among the most interesting crabs are the hermit crabs (fig. 37),
familiar to all who know the seashore. There are numerous species of
these crabs, all of which have the habit of carrying about with them, as
a protective covering into which to withdraw, the spiral shell of some
gastropod mollusc. The abdomen of the crab remains always in the cavity
of the shell; the head and thorax and legs project from the opening of
the shell, to be withdrawn into it when the animal is alarmed or at
rest. The abdomen being always in the shell and thus protected loses the
hard body-wall, and is soft, often curiously shaped and twisted to
correspond to the cavity of the shell. It has on it no legs or
appendages except a pair for the hindmost segment which are modified
into hooks for holding fast to the interior of the shell. As the hermit
crab grows it takes up its abode in larger and larger shells, sometimes
killing and removing piece-meal the original inhabitant. Some hermit
crabs always have attached to the shell certain kinds of sea-anemones.
It is believed that both crab and sea-anemone derive advantage from this
arrangement. The sea-anemone, which otherwise cannot move, is carried
from place to place by the crab and so may get a larger supply of food,
while the crab is protected from its enemies, the predaceous fishes, by
the stinging threads of the sea-anemone, and also perhaps by the
concealment of the shell its presence affords. This living together by
two kinds of animals to their mutual advantage is called commensalism or
symbiosis (see Chapter XXX). The hermit crabs are not true crabs, but
are more nearly related to the crayfishes and shrimps than to the true
broad-bodied, short-tailed crabs.
=Barnacles.=--TECHNICAL NOTE.--Specimens of barnacles may be got
readily from the tide rocks or from piles in a harbor. Interior
schools should have, if possible, specimens preserved in alcohol or
formalin for examination. The "shells" of acorn (sessile) barnacles
may often be found on oyster shells (get at restaurants).
Crustaceans which at first glance are hardly recognizable as such are
the stalked or sessile barnacles (fig. 37) which live fixed in great
numbers on the rocks between the tide lines, or on the piles
supporting wharves, or on the bottom of ships or even on the body-wall
of whales and other ocean animals. In the stalked forms the stalk is a
flexible stem or peduncle covered with a blackish finely-wrinkled skin
bearing at its free end the greatly modified body of the barnacle.
This body is enclosed in a sort of bivalved shell or carapace formed
by a fold of the skin and stiffened by five calcareous plates. Within
this curious shell is the compact, rather worm-like body-mass, showing
little or no indication of segmentation. The legs, of which there are
usually six pairs, are much modified, being long, feathery, and
divided nearly to the base. These feathery feet project from the
opened shell when the animal is undisturbed, and waving about in the
water catch small animals which serve as the barnacle's food. When
disturbed the barnacle withdraws its feet and closes tightly its
strong protecting shell. The acorn-barnacles have no stalk, but look
like a low bluntly-pointed pyramid, this appearance being due to the
converging arrangement of six calcareous plates in its body-wall.
The barnacles present several unusual conditions with regard to the
internal organs. They have no heart nor any blood-vessels; most of the
species are hermaphroditic; and there are other indications of a
degenerate condition. This degeneration of the barnacles is due to
their fixed life, the results of which are like those of a parasitic
life. The young barnacles when hatched from the egg are free-swimming
larvæ as with the other Crustacea. They finally attach themselves and
undergo the changes, some of them of degenerative nature, which
produce the body-structure of the adult. It was long a belief among
many people that the barnacle produced the barnacle goose. Pictures in
ancient books show the young barnacle geese issuing from the opened
shell of the barnacle. The early naturalists believed barnacles, on
account of the shell, to be a kind of shell-fish or mollusc, but when
their development was thoroughly worked out, it became evident that
they belong to the Crustacea.