Harold Heath David Starr Jordan.

Animal forms: a second book of zoology online

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from forty to sixty feet. There are many stories of their
great strength and of their voluntarily attacking people
and even overturning boats, but the latter are in almost
every case sailors^ yarns.

In their external organization the cephalopods have
little to remind one of any of the preceding mollusks, and
their internal structure shows only a distant resemblance.
In the Octopi (Fig. 52) the shell is lacking ; in the squid it
is called the pen, and consists of a horn-like substance with-
out any lime deposit ; in the cuttlefishes it is spongy and
plate-like, and is a familiar object in the shops ; and, finally,



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90 ANIMAL FORMS

in the nautilus it is coiled and of considerable size, and, un-
like that of any other cephalopod, it is carried on the out-
side of the animal. Interiorly it is divided by a number of
partitions into chambers, the last one of which is occupied
by the animal.

The alimentary canal shows some resemblance to that
of other mollusks, but, as in the case of the other systems
of the body, it possesses a far higher state of development.
The mouth is situated in the center of a circle of arms,
which in reality are modified portions of the foot, and is
furnished with two parrot-like jaws. From this point the
esophagus leads back into the body mass to the stomach,
which with the liver and intestine are sufficiently like
those of the clam and snail to require no further comment.

Eespiration is effected by the skin to a certain extent,
but chiefly by two gills (four in the nautilus), and the cir-
culatory system, which conveys the blood to and from these
organs and over the body with its complex heart, arteries,
capillaries, and veins, is more highly developed than in
any other invertebrate.

As might be expected in animals with so great sagacity
and cunning, the nervous system of the sense-organs reach
a degree of development but little short of what we find in
some of the vertebrates. The chief part of the nervous
system is located in the head, protected by a cartilaginous
skull, a very rare structure among invertebrates ; and while
the different ganglia may be recognized in a general way
and be found to correspond to a certain extent to those
of foregoing mollusks, they are so largely developed and
massed together that it is impossible at present to under-
stand them fully. From this point nerves pass to all
regions of the body, to the powerful muscles, the viscera,
and the organs of special sense, controlling the complex
mechanism in all its workings.

There is no doubt that the cephalopods see distinctly
for considerable distances, and a careful examination of



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MOLLUSKS 91

the eye of the squids and cuttlefishes has shown them
to be remarkably complex and in many respects to be
constructed upon much the same plan as those of the
yertebrates. As to the other senses not so much is known,
but undoubtedly many species of cephalopods are possessed
of a shrewdness and cunning not shared by any other
invertebrates, save some of the insects and spiders, and are
vastly more highly organized than their moUuscan rela-
tives.

91. How species originate.— We have now examined a
considerable portion of the animal kingdom, tracing its
members from their simplest beginnings as single cells,
through the formation of colonial types, and up through
the sponges, coelenterates, worms, and moUusks. It is im-
portant once more to note that they all perform the func-
tions concerned in nutrition and reproduction, and only
these. The differences which exist are those of structure.
The Hydra and the clam, for example, perform the same
duties, but their bodily apparatus differs widely, and the
completeness and perfection of the work varies accord-
ingly. The more the work to be performed by an organ-
ism is divided up among especially adapted organs, so that
each of the latter has, as far as possible, only one thing to
do, the higher is the organism.

As stated earlier in the account, it is believed that the
more complex animals arose from the simpler ; that if we
could trace the history of any of the great groups back
toward their first beginnings, we would find them all to
have originated from one ancestral form, that in turn owes
its descent from yet simpler forms.

Let us see something of how this has come about. We
all know that vast numbers of young are born into this
world which never come to maturity. It is said that if all
the young of the codfish were to live their allotted time,
it would be less than fifteen years before the sea would
become literally packed with them. Numerous enemies,



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92 ANIMAL FORMS

the lack of food, and other agencies annihilate the larger
part. We also know that no two offspring are exactly
alike. They exhibit individual differences. One bird may
have a larger bill than another of the same brood which
excels in length of wing. As noted above, all the offspring
will not attain maturity. Those best adapted to their sur-
roundings will have the best chances of survival. The
increased length of bill or wing may be slight, but it may
be just this amount which enables the bird to probe deeper
or fly farther and thus secure the requisite amount of food.
A premium is placed on length of wing or bill generation
after generation, with the result that a long-billed species
arises distinct from the long-winged which trace their
ancestry back to the same parents. It is the same prin-
ciple which enables the breeder to increase the swiftness
of the race-horse and the strength of the draft-horse, or
the gardener to develop from the wild rose the great num-
ber of widely different varieties. In the same way other
slight peculiarities over very many generations may en-
able other forms to gradually adapt themselves to still dif-
ferent modes of life. Thus vast numbers of organisms
gradually become modified in form and complexity, and
are adapted to lives which insure them a comparative
degree of safety and less competition with other species.



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CHAPTER IX

ARTHBOPODS. CLASS CRUSTACEA

92. General characters. — In the Arthropods, that is, the
crabs, lobsters, shrimps, insects, spiders, and a vast host
of related forms, the body is bilaterally symmetrical, and
is composed of a number of segments arranged in a series,
as in the earthworm and other annelids. A hornlike cu-
ticle, sometimes called the shell, bounds the external sur-
face — in early life thin and delicate, but later relatively
thick, and often further strengthened by lime salts. Along
the line between the segments this coat of mail remains
thin and forms a flexible joint. Appendages also are borne
on each segment, not comparatively short and fleshy out-
growths like the lateral appendages of many of the worms,
but usually long and jointed (hence the name Arthropod,
meaning jointed foot), and variously modified for many
different uses.

93. Classification. — The species belonging to this group
outnumber the remainder of the animal kingdom. Their
haunts also are most diverse. Some are adapted for lives
in the sea and fresh water, others for widely different sit-
uations on land, and a great number are constructed for a
life on the wing. A certain resemblance exists among them
all, but the modifications which fit them for their different
habitats are also profound, and have resulted in the division
of the Arthropods into five classes. The first class {Crus-
tacea) contains the crayfish, crabs, etc. ; the second ( 07iy-
chophora) includes the curious worm-like peripatus (Fig.

98



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94 ANIMAL FORMS

66); the third {Myriapoda^ meaning myriad-footed) em-
braces the centipede and "thousand-legs" ; the fourth (/w-
seda) contains the insects ; and the fifth (Arachnida) in-
cludes the scorpions, spiders, and mites.

94. The Crustacea.— The number of species of crusta-
ceans is estimated to be about ten thousand, and while the
greater number of these are marine, many are found in
fresh water and a few occur on land. In size they range
from almost microscopic forms to the giant crabs and
lobsters. They differ also in shape to a remarkable degree,
but at the same time there is a decided resemblance through-
out the group, except in those species which have become
modified by a parasitic habit. The characteristic external
skeleton is invariably present, and gives evidence of the
deep internal segmentation of the body. In the simple
Crustacea this is very apparent, but in the higher forms it
is usually more or less obscured, owing to the fusion of some
of the different segments, especially those of the head, as in
the crayfish (Fig. 59).

The class of the Crustacea is subdivided into two sub-
classes {Entomostraca and Malacostraca)^ the first containing
the fairy-shrimps {Branchipus^ Fig. 53) and their allies, the
copepods (such as Fig. 54), the barnacles (Fig. 55), and a
number of other species. In their organization all are com-
paratively simple, usually small, and the appendages show
relatively little specialization. The other subclass contains
the more highly developed and usually large-sized Crustacea,
among which are the shrimps, crayfishes, lobsters, crabs,
and a number of other forms.

95. Some simple Crustacea. — While the members of the
first subclass are minute and inconspicuous, several species
are often remarkably abundant in our small fresh-water
pools. Among these is the beautifully colored fairy-shrimp
{BranchipuSy Fig. 53), with greatly elongated body and
leaf -like appendages, whose relatively simple character leads
the zoologist to think that they are among the simplest



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ARTHROPODS. CLASS CRUSTACEA 95

Crustacea, and in several points resemble the ancestral form
from which all the modem species have descended. Some
nearly related forms are provided with a great fold of the
body-wall, which may almost completely conceal the animal
from above, or it may be formed like a bivalve clam-shell,
within which the entire body may be withdrawn. This



latter character is also found in the water-fleas {Daphnia)j
very much smaller forms, and sometimes occurring in mil-
lions on the bottoms of our ponds and marshes. They are
readily distinguished from the fairy-shrimp by the short-
ness of the body, the small number of appendages, and by
their habit of using their antennae as swimming organs,
which gives to their locomotion a jerky, awkward character.
96. Cyclops and relatives. — Cyclops (Fig. 54), the repre-
sentative of a number of lowly forms belonging to the order
of Copepods, is one of the commonest fresh-water Crustacea.
The forward segments of the spindle-shaped body are cov-
ered by a large shield or carapace, the feet are few in num-
ber, and, like its fabled namesake, it bears an eye in the
center of the forehead. Nearly related species are also re-
markably abundant at the surface of the sea, at times occur-



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96



ANIMAL FORMS




ring in such vast numbers that they impart a reddish tinge
to the water over wide areas, and at night are largely re-
sponsible for its phos-
phorescence. Many oth-
ers are parasitic in their
habits, and scarcely a
salt-water fish exists but
that at one time or an-
other suffers from their
attacks. On the other
hand, many fresh- and
salt-water fishes depend
upon the free-swimming
forms for food, and
hence, from an economic
point of view, they are
highly important organ-
isms.

97. Barnacles. — The
parasitic habit and the
lack of locomotion has
also produced marvelous
changes among the bar-
nacles, so great that
originally they were
placed among the mol-
lusks; and as with the parasitic copepods, their true posi-
tion was only known after their life-history had been de-
termined. In the goose-barnacles * the body, attached by
a fleshy stalk to foreign objects, is enclosed by a tough
membrane, corresponding to the carapace of other Crus-
tacea, in which are embedded five calcareous plates. This




Fig. 54.— Cyclops, e. «., eggs ; i, intestine ; op,
reproductive organ.



* So called because of the belief, which existed for three hundred
years prior to the present century, that when mature these animals
give birth to ^eese,



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ARTHROPODS. CLASS CRUSTACEA 97

is open along one side, and allows the feather-like feet to
project and produce currents in the surrounding water
which brings food within reach. In the acorn-barnacles
(Fig. 55) the stalk is absent, and the body, though possess-



FiQ. .'>5.— Barnacles. Acorn-barnacles chiefly in lower part of flgnre ; goose-barnacles
above. Natural size.

ing the same general character as the goose-barnacles, is
shorter, and enclosed in a strong palisade consisting of six
calcareous plates.

The larger number of barnacles attach themselves to
the supports of wharves, the hulls of ships, floating tim-
bers, the rocks from the shore-line down to considerable
depth, and a few species occur on the skin of sharks and
whales. On the other hand, there are several species which
are parasitic, and in accordance with this mode of life ex-
hibit various degrees of degeneration. In the most extreme



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98 ANIMAL FORMS

cases {Sacculina) the sac-like body, attached to the abdo-
men of crabs, is entirely devoid of appendages and any
signs of segmentation. A root-like system of delicate fila-
ments extends from the exposed part of the animal into
the host and absorbs the necessary nutriment. The mouth
and alimentary canal are accordingly absent — in fact, the
body contains little but the reproductive organs and a very
simple nervous system.

98. Structure. — In the internal organization of these
smaller crustaceans many differences may be noted, though
they are usually less profound than the external. Ordi-
narily the alimentary canal is a straight tube passing
through the body, and is provided with a pouch-like
stomach, and a more or less clearly defined liver. In
all, except the parasitic species, the external mouth-ap-
pendages masticate the food, and in a very few of the
above-described groups it may be further ground between
the horny ridges on the stomach-walls. After this pre-
liminary treatment it is subjected to the action of the
digestive juices, and when liquefied is absorbed into the
body. Here it is circulated by a blood-system of widely
different character. In many cases definite arteries and
veins are absent. The blood courses through the body in
the spaces between the different organs propelled by the
beating of the heart, which it is made to traverse. In
Cyclops (Fig. 54) even the heart is absent, and the blood
is made to circulate by contractions of the intestine. In
most of these smaller Crustacea considerable oxygen is ab-
sorbed through the body-wall ; but in several species, for
example, the fairy-shrimp (Fig. 53), special gills are devel-
oped on the appendages of the body.

99. Multiplication. — Among the Crustacea thus far con-
sidered the males are usually readily recognized owing to
their small size. The females also are usually provided
with brood-pouches in which the developing eggs are pro-
tected. In almost every case the young are born in the



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ARTHROPODS. CLASS CRUSTACEA



99



form of minute larvae, provided with three pairs of append-
ages, a median eye (Fig. 56), and a firm external skeleton
or cuticle. This latter prevents the continuous growth of
the larvae or nauplius, and every few days it is thrown off,
and while the new one is forming the body enlarges. Dur-
ing this time new appendages are developed, so that after
each moult the young crusta-
cean emerges less like its
former self and more and more
like its parents. In the bar-
nacles, after several moults
have taken place, the young
become permanently attached
by means of their first anten-
nae, their thoracic feet change
into feathery appendages, and
several other changes occur.
In some of the parasitic bar-
nacles (Sacculina) the larva
attaches itself to a crab, throws
off its various appendages, and,
after other great degenerative
changes, enters its host. For
a time, therefore, their development is toward greater com-
plexity, but the later stages constitute a retrograde meta-
morphosis,

100. More complex types. — The larger, more useful, and
usually more familiar Crustacea belong to the second divi-
sion (subclass Malacostraca). It comprises such animals as
the shrimps, crayfish, lobsters, crabs, and a number of other
forms which are at once distinguished from the preceding
by the constant number of segments composing the body.
Of these, five constitute the head, eight the thorax, and
seven the abdomen. The head segments are always fused
together, and with them one or more thoracic segments
unite to form a more or less complete cephalothorax. Also,




Fig. 56.— Development of a barnacle
{Lepaa). a, larva ; b, adult.



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100 ANIMAL FORMS

some of the head segments give rise to a great fold of the
body-wall, the carapace, which extends backward and covers
all or a part of the thorax, with which it may firmly unite,
as in the crayfish. The appendages are usually highly spe-
cialized, and are made to perform a variety of functions.

101. The shrimps. — Among the simplest of these are the
opossum-shrimps (Fig. 57) and their relatives, small trans-




FiQ. 57.— The opossum-Bhrimp (Mysis americana).

parent creatures often seen swimming in great numbers at
the surface of the sea or hiding among the seaweeds along
the shore. In general appearance they resemble crayfishes
or prawns, but are readily distinguished by the two-branched
thoracic feet. This "split-foot" character also occurs
among many of the preceding Crustacea, and is generally
a badge of low organization, tending to disappear in the
more highly organized forms. In this and other respects
the shrimps are especially interesting in their relation to
the preceding Crustacea, and in the fact that they may
closely resemble the ancestors of the modern prawns (Fig.
58), lobsters, crayfishes, and crabs.

102. Crayfishes and lobsters. — The last-mentioned spe-
cies and their allies, usually large and familiar forms, con-
stitute a group known as the decapods (meaning ten feet),
referring to the number of thoracic feet. Among the mem-
bers of this division probably none are more familiar than
the crayfishes, which occur in most of the larger rivers and
their tributaries throughout the United States and Europe.
It is their habit to remain concealed in crevices of rocks



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ARTHROPODS. CLASS CRUSTACEA 101

or in the mouths of the burrows which they excavate, and
from which they rush upon the small fish, the larvae of



many animals, and other equally defenseless creatures
which constitute their bill of fare. In turn they are
eagerly sought by certain birds and four-footed animals, and,
especially in France,
are extensively used for
food by man.

Closely related to
the crayfishes and dif-
fering but little from
them structurally are
the lobsters. In this
country they are con-
fined to the rocky coasts
from New Jersey to
Labrador, living upon
fish, fresh or otherwise,
various invertebrates,
and occasionally sea-
weeds. Far more than
the crayfish, the lobster
is in demand as an arti-
cle of food. By the aid

of nets or various traps Fia. 59.— The crayfish (Astacus),



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102 ANIMAL FORMS

millions are caught each year, and to Buch an extent, has
their destruction proceeded that in many places they are
well-nigh exterminated. At the present time, however, leg-
islation, numerous hatcheries, and a careful study of their
life habits is doing much to better matters and inciden-
tally to put us in possession of many interesting zoological
facts along this line, some of which will be mentioned later.
Frequently the prawns, especially the larger ones, and a
spiny lobster (Palinurus), are mistaken for crayfishes or
lobsters, but they differ from them in the absence of the
large grasping claws.

Along almost any coast some of these animals are to be
found, often beautifully colored and harmonizing with the
seaweeds among which they live, or so transparent that
their internal organization may be distinctly seen. Farther
out at sea other species swim in incredible numbers, feed-
ing upon minute organisms, and in turn fed upon by numer-
ous fishes and whales ; and, especially on the Pacific coast,
shrimp-fishing is an important industry.

103. The hermit-crabs.— The last of these long-tailed
decapods is the interesting group of the hermit-crabs,
which occur in various situations in the sea. In early life
they take possession of the empty shell of some snail, and
the protected abdomen becomes soft and fiabby, while the
appendages in this region almost completely disappear.
The front part of the body, on the other hand, continually
grows in firmness and strength, and is admirably adapted
for the continual warfare which these forms wage among
themselves. As growth proceeds the necessity arises for a
larger shell, and the crab goes "house-hunting" among the
empty shells along the shore, or it may forcibly extract the
snail or other hermit from the home which strikes its fancy.

Many of the hermit-crabs enjoy immunity from the
attacks of their belligerent relatives by allowing various
hydroids to grow upon their homes. Others attach sea-
anemones to their shells or to one of their large claws,



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ARTHROPODS. CLASS CRUSTACEA lOS

which they poke into the face of any intruder. While
the anemones or hydroids are made to do valiant service



Pig. 60.— Hermit-crab {Fagurus bemhardus) in snail sliell covered with Hydractinia.

with their nettle-cells, they also enjoy the advantages of
a large food-supply which is attendant upon the free ride.

104. The crabs. — The most highly developed Crustacea
are the crabs or short-tailed decapods which abound between
tide-marks alongshore, and in diminishing numbers extend
to great depths. The cephalothorax is usually relatively
wide, often wider than long, and the greatly reduced abdo-
men is folded against the under side of the thorax. Corre-
lated with the small size of the abdomen, the appendages
of that region disappear more or less, but the remaining
appendages are similar to those of the crayjSsh or lobsters.
All these different parts, however, are variously modified in
each species to fit it for its own peculiar mode of life. In
some forms, such as the common cancer-crab (Fig. 61), the
legs are comparatively thick-set and possessed of great
strength, enabling them to defend themselves against most
enemies. On the other hand, there are the spider-crabs
with small bodies and relatively long legs, withal weak, and
8



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104 ANIMAL FORMS

yet so harmonizing with their surroundings that they are
as likely to survive as their stronger relatives. In this



Fig. 61.— Kelp-crab (Hpialttts productus) in upper part of figure ; to the right the
edible crab {Cancer productus), and the shore-crab {Pugettia richii).

connection it is interesting to note that the giant crab of
Japan, the largest crustacean, being upward of twenty feet
from tip to tip of the legs, is a spider-crab, constructed on



Fig. 62.— The fiddler-crab {Gelasimus). Photograph by Miss Mary Rathbun.

the same general pattern as our common coast forms.
Between these two extremes numberless variations exist,



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ARTHROPODS. CLASS CRUSTACEA 105

some for known reasons, but more often not readily under-
stood. And not only does the form vary, but the external
surface may be sculptured or beset with spines or tubercles
which frequently render the animal inconspicuous amid its
natural surroundings. Such an effect is heightened by the
presence of sponges, hydroids, and various seaweeds which
the crab often permits to gather upon its body.

105. Pill-bugs and sandhoppers. — Finally there remain the
groups of the pill- or sow-bugs (Isopods) and the sand-fleas
or sandhoppers (Amphipods). In the first of these the
body is usually small and compressed, the thorax more or
less plainly segmented, and the seven walking (thoracic)
legs are similar. In the female each leg bears at its base a
thin membranous plate which extends inward and hori-



FiG. 63.— Isopod or pill-bug {Porcellio laevis).

zontally, thus forming on the under side of the body a


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Online LibraryHarold Heath David Starr JordanAnimal forms: a second book of zoology → online text (page 7 of 17)