Several groups of the lower animals are collectively known as Worms,
though most of these groups are but remotely related. Ringed worms
are elongated creatures in which the body is made up of a
considerable number of rings or segments, most of which are, on the
whole, much alike. There is often a well-marked head, but no
distinct thorax and abdomen, as in an insect or crayfish. Two
subdivisions are recognized: (a) Bristle-worms (_Chætopoda_) and (b)
Leeches (_Discophora_).
=Bristle-worms= include a host of marine worms, together with some that
live in fresh water, and also the earthworms. Their average characters
are best understood by examining one of the commonest shore-worms, known
as the sea-centipede (_Nereis_). Here the segments are very clearly
seen, and almost every one of them bears a pair of unjointed conical
foot-stumps, used for crawling.
Imbedded in the foot-stumps of the sea-centipede are bundles of strong
bristles, which give a hold on the underlying surface and prevent
slipping. The head-region is fairly distinct, and bears a number of
feelers of various kinds, as well as four simple eyes. Sea-centipedes
and many of their allies are highly carnivorous, and seize their prey by
means of a pair of horny jaws which can be protruded at will.
=Earthworms= are found in all parts of the world, though naturally they
do not thrive in arid tracts; and their effect upon the fertility and
drainage of the soil can hardly be calculated. Burrowing into the
ground, they cast up the earth they have swallowed, and so pursue a
constant and thorough system of ploughing. Though eyeless, they evade
the light and only come out of their burrows at dusk, often remaining,
even then, with their tails in the holes and their bodies working round
and round.
Darwin long since demonstrated, the earthworm is one of the farmers’
best friends. Its burrows drain and aerate the soil, while the earth
which has passed through its body is finely divided and constantly being
brought to the surface from lower levels.
Not far from the front end of an earthworm a thickening will be seen,
often erroneously supposed to be the result of injury. From it exudes a
fluid which hardens into the egg-cases.
=Leeches= live in the sea, fresh water, or even in damp, tropical
forests. The flattened body of the leech is divided by grooves into a
number of narrow parts, several of which go to make up a segment.
Foot-stumps and bristles are entirely absent, and progression is
effected by means of suckers, one at each end. They effect a looping
movement, but the animal can also swim by undulations of its body. The
freshwater leech is a bloodsucking parasite. The mouth is situated in
the middle of the front sucker, which serves to fix the animal to its
victim. Three saw-edged jaws are then brought into play, a three-rayed
cut being made, and a fluid poured out which prevents the blood from
clotting. Digestion is slow, and the food is stored in a large crop,
drawn out into numerous pairs of pouches. The head possesses eye-spots,
but no feelers.
=ANIMALS LIKE PLANTS= (_Cœlenterata_)
of which sea-anemones, corals, and jelly-fishes are examples, are
distinguished by the ray-like symmetry of starfishes and their kind,
though here, as a rule, it is more perfect. In structure they are
much simpler, than any of the animals so far considered. For such a
creature is to all intents and purposes simply a stomach, the wall
of which is made up of two layers of cells, one (_ectoderm_)
external, and the other (_endoderm_) internal. In higher animals a
third layer (_mesoderm_) is interposed.
=Sea Anemones= are common between tides and lower on all coasts. They
are cylindrical animals, with a mouth surrounded by tentacles at one
end. Inside there is a single cavity which serves as a stomach and whose
branches run to all parts of the body, thus distributing the food like a
blood vessel. The colors, especially in the tropics, are variable, and
often gorgeous.
=Coral-polyps= are closely related to sea-anemones, but differ from them
by secreting a hard, limy skeleton in the base of the body. They are
either simple or compound. The well-known mushroom coral may be taken as
an example of the former. Its skeleton is a shallow cup, exhibiting
numerous radiating plates. If we look at the upper surface of such a
coral in the living state we shall see a mouth surrounded by circlets of
tentacles, much as in a sea-anemone.
A compound coral consists of a number of individuals, relatively small
in size, connected together by a common flesh, and formed by the budding
or splitting of a single original polyp, the results of the process
remaining united.
Many corals branch, while others form compact masses, as in the kind
above described, and also in the brain coral, where the boundaries
between the individuals are not clearly marked.
Corals are widely distributed, some living even in cold latitudes, and
others on the floor of the deep sea. Coral reefs, however, made up of
the skeletons of such animals, are only found in the warmer parts of the
ocean, where the water is clear, particularly favorable conditions being
afforded by the Pacific and Indian Oceans. (See also Coral Reefs and
Islands).
=Jelly-fish= (_Medusæ_).--All agree in having a more or less bell or
umbrella shaped body, with a proboscis hanging down in the place of the
handle of the umbrella or the tongue of the bell. The mouth is at the
end of the handle and leads into a stomach which divides and sends out
branches, like the ribs of the umbrella, to the margin. The common name
is due to its gelatinous consistency. Most of the species start in life
as buds from attached animals, which later separate and henceforth lead
a free existence, swimming by opening and closing the bell.
=SPONGES= (_Porifera_)
are animals of peculiar structure, which resemble zoophytes in many
respects, but possess neither tentacles not thread-cells. Some are
simple, but most of them are compound. A simple sponge may be
compared to a cup or vase with a wall perforated by numerous small
holes, through which currents of sea-water stream into the central
cavity, to make their exit by the main opening. They are set up by
ciliary action.
=Venus Flower-basket.=--In the majority of cases the skeleton of a
sponge is mostly or entirely made up of sharp needles of lime or flint,
which may be welded together, as in this form. Often the opening of the
vase is provided with a convex perforated covering. Another elegant form
is the Glassrope Sponge native to the Japanese seas. It is rooted in the
mud by a bundle of long, glassy spicules, which are slightly twisted.
Most sponges are marine, and, despite their fixed habit and apparent
helplessness, are pretty free from the attacks of most other creatures,
partly because of the innumerable sharp spicules they contain, and
partly because their taste and smell are unpleasant. These deterrent
qualities are often associated with bright warning colors, generally
red, yellow or orange.
Most of us little realize that the sponges we see or use daily are in
reality dead animals.
[Illustration: =CROSS-SECTION OF LIVING SPONGE=
=THE SPONGE IN ACTION=
The arrows show how the water enters by the small pores, to pass out by
the large openings. Food is thus brought to the cells which line the
channels. At the right the currents of water are seen passing from the
outer openings, as seen under a powerful microscope.]
[Illustration: =THE BORDER-LINE BETWEEN ANIMAL AND PLANT LIFE=
On the left are represented highly magnified animal skeletons of a class
of Protozoa called Radiolarians. Most of them are under one-twenty-fifth
of an inch in size. Millions upon millions of these little shells are
found upon the floors of the ocean, and upon its shores. They are
marvels of form and color--so wonderful, indeed, that man with all his
skill cannot imitate them. When alive they consist of but a single cell,
and live in colonies with the plant forms, called algæ, pictured on the
right. The algæ are also single-celled, and of rare beauty of form, and
the strange association of the simplest of animal forms with the
simplest of _plant_ forms has up to the present time proved the supreme
enigma of science.]
_PROTOZOA OR SIMPLEST FORMS OF ANIMAL LIFE_
=ANIMALCULES, AMOEBA, RADIOLARIA, FLAGELLATES, CILIATES=
=ANIMALCULES= (_Protozoa_)
In botany we find that the lowest plants are mostly of a microscopic
size, and unicellular--that is, consisting of a single cell or
structural unit, essentially a fragment of living matter
(protoplasm), part of which is specialized into a nucleus. The
lowest animals are also unicellular, and the popular term
“animalcule”--a little animal--has reference to their diminutive
size. One of the simplest known cases is afforded by the
=Amœba= (_Gr._ “change”), a name given to a number of the simplest
animals or protozoa. The simplest form which the observer will meet is a
naked lump of jelly-like protoplasm constantly flowing into new shapes.
=Ray Animalcules= (_Radiolaria_) are forms which resemble the members of
the last group in some respects, but are more complex in structure, with
shells composed of a latticework of flinty matter. These shells cover
large tracts of the floor of the deeper parts of the ocean (limy shells
dissolve before getting so far), and make up “radiolarian oozes,” such
as Barbadoes earth.
=Flagellates= are immensely numerous animalcules with a body of definite
shape covered by a membrane. Swimming is effected by a slender thread of
flagellum (Latin for whiplash) of living substance, which executes
whip-like movements. They are common in ponds and ditches, where it
often makes up a green scum. A mouth is situated at the base of the
flagellum, and at this end there is also a red eye-spot. Some
flagellates bear more than one flagellum, many are fixed, and the
colonial condition is common. The exceedingly minute animalcules which
swarm in putrid fluids and are vaguely known as “monads” belong to this
group.
=Ciliates= like flagellates, are invested in a firm membrane, and
therefore of definite form. Instead of flagella, they possess cilia,
short threads of living substance which are associated in large numbers,
and alternately bend and straighten in a rhythmic fashion, bringing
about locomotion in free species, or setting up currents in the water in
fixed ones.
Despite their apparent insignificance, certain animalcules, by virtue of
their almost imperishable skeletons, are among the most important
agencies which have built up the crust of the earth. The surface of the
sea is largely inhabited by Radiolarians and Foraminifera, the former
preponderating in cold, the latter in temperate and tropical waters. As
they die, their skeletons sink to the bottom and form mud or ooze, which
through time and pressure becomes consolidated into rock.
[Illustration: It is generally believed that sheep were the very first
of all domesticated animals. Doubtless because they supplied him with
food and clothing and by reason of their gentleness, they were selected
by man as his first animal associate.]
_DOMESTICATED ANIMALS_
=DOMESTICATED MAMMALS:= Alpaca, Ass, Camel, Cat, Cattle, Dog,
Elephant, Gayal, Goat, Guinea Pig, Horse, Llama, Rabbit, Reindeer,
Sheep, Swine, Yak, Zebu. =DOMESTICATED FISH:= Carp, Goldfish.
=DOMESTICATED BIRDS:= Canary, Chickens or Fowls, Duck, Guinea,
Goose, Ostrich, Parrot, Peacock, Pigeon, Swan, Turkey. =DOMESTICATED
INSECTS:= Bee, Cochineal, Silkworm Moth.
Domestic animals are those kept for the use or companionship of man.
When studied in their relation to the animal kingdom as a whole it is
readily seen that they belong to the highest groups of animals; but the
actual process of original domestication is unknown to us. It is also
very evident that the origin of some of the domesticated groups
themselves is very obscure. In general it may be said that only when a
distinct breed has been produced by human interference may we call the
result domestication.