ANNULOSA, OR WORMS.
THE Annulosa, which are the lowest grade of articulated animals, consist
of four distinct orders: the Entozoa, which are muscle and intestine
parasites; the Turbellariæ, fresh and salt-water animals covered with
cilia; the Annelida, or Worms; and the Rotifers, or Wheel animalcules.
_Entozoa._
There are three genera and numerous species of Entozoa. Every animal has
one or more species peculiar to itself; fourteen infest the human race.
They have a soft, absorbent body of a white or whitish colour, in
consequence of being excluded from light, and living as they do by
absorbing the vitalized juices of the animals they infest. Their
nutritive system is in the lowest state of development; yet there are
some of a higher grade. All are remarkable for their vast
productiveness.
The Tænioïdæ, which belong to the inferior group, are intestinal,
many-jointed worms, which have neither mouth nor digestive organs; and
what is called the head has only hooks and suckers to fasten it to the
internal membrane of the animal at whose expense it lives. The common
Tænia, or Tape-worm, sometimes ten feet long, which is the type of the
order, has four suckers and a circle of hooklets round a terminal
proboscis to attach it to its victim. Though destitute of the organs of
nutrition it is extremely prolific, for each segment of its long flat
body is a reproductive monœcious zooid, which forms and lays its own
eggs exactly as if it were a single independent animal, thus furnishing
a very remarkable instance of the law of irrelative repetition, which is
a series of organs performing the same functions independently of one
another. Two pairs of canals containing a clear colourless liquid extend
throughout the body of the worm.
Bags, or vesicles called cysts, had been found in the glands and muscles
of various animals, afterwards discovered to contain young worms, which
attain their perfect development within such creatures as eat the flesh
containing the cysts. Under circumstances so unprecedented, it required
no small skill and patience to determine the life-history of these
singular creatures. The cysts differ in size and form according to the
genera, and are embedded singly or in groups in the flesh of their
victim, on whose ready prepared juices they live.
The greater number of the Tænia genus begin their lives as sexless
cysted larvæ, and on entering their final abode, segments are
successively added till the worm has arrived at its adult state. The
tape-worm of the cat has its origin in the encysted larvæ found in the
livers of the mouse and rat. One species of Entozoa, while in its
primary state, inhabits the stomach of the stickleback, and only comes
to perfection within the aquatic birds that feed on this fish. Another
species infests the livers of the salmon tribe, and gets its perfect
form in the pike and perch.
Sheep and the hog are more tormented with cysted worms than any other
domestic animals used for food. If introduced into the human intestines
by eating raw ham or sausages, the larvæ soon acquire the perfect form.
The eggs of the Tænia may be introduced into the human or animal
stomach; for dogs and other carnivora which eat raw unwholesome meat are
infested by full grown tænia, which fix themselves to their entrails by
their hooks and suckers, while at the same time egg-bearing segments
separate successively from their posterior extremity, and being voided
scatter the eggs far and wide on land and in water.
The young of some Entozoa undergo various transformations, as those of
the Distoma of the Lymnæa. When full grown that entozoön is like a sole,
flat, broad, and long, with a kind of head at the broad end, and two
suckers on its under-surface, in one of which there is a pore serving as
a mouth, whence an alimentary canal extends, which spreads in branches
almost throughout the whole body. This animal has a filamentary nerve
round its gullet, from which minute fibres pass to the mouth, and two
filaments extend backward on each side as far as the second sucker. The
eggs which occupy the whole margin of the body are developed into worms,
each of which seems to be merely a mass of structureless cells enclosed
in a contractile case. By a second change each of these cells is
transformed into a freely swimming ciliated zooid endowed with eyes.
Having escaped from their contractile case, they remain for a time in
that state, and then imbed themselves in the mucus on the foot of the
fresh-water mollusk Lymnæa, or pond snail, where they are transformed
into true Distomata, and ultimately enter into the body of the Lymnæa
itself, where they lose their eyes and cilia, which are no longer of use
in their dark and permanent abode. The Fluke found in the livers of
sheep that have the rot is a Distoma.
The Nematoid order, or thread-worms, that live in the muscles of men and
animals, are long, smooth, and cylindrical, with a structureless skin
covering layers of longitudinal and circular fibres, by means of which
they can stretch and contract themselves. They are generally pointed at
both ends with a mouth at one extremity and an orifice at the other. The
Filariæ are slender, sometimes of great length, as the Guinea worm,
which varies in length from six inches to two, eight, or even twelve
feet. In Persia they are believed to be introduced into the system by
drinking water in which their eggs have been deposited. This worm may
grow in the muscles of a man to the size of five or six feet without
giving much annoyance, but when its head bores through the skin it
produces a painful sore unless extracted. In Persia, where the worm is
common, the natives seize it by the head, draw it carefully out, and
wind it round a bit of wood, an operation which may require several days
to accomplish. It has a numerous viviparous progeny, which come out
through the mouth. There are certain very small slender species of
Filaria which attack the eyes both of men and horses; some bury
themselves close to the eye, and a very minute kind enters the ball
itself.
The Ascaris lumbricoïdes, a common intestinal thread-worm of the hog,
ox, and the human race, is sometimes of great length. The sexes are
distinct, and their fertility enormous. The ovaries are two tubes
sometimes several feet long, in each of which the eggs are arranged in
whorls round a central stem, like the flowers of a plantago. By counting
the number of microscopic eggs in a whorl, and the number of whorls, Dr.
Eschricht ascertained that in a full grown female the average number of
eggs amounted to sixty-four millions. In this species of worm the embryo
is not developed from the egg while within the victim, so that most of
the eggs perish.
Different species of Anguillulæ, which are minute eel-like worms slender
as a hair, inhabit the alimentary canal of fresh-water snails, frogs,
and fishes, but many species are not parasitic. These are often united
in swarming masses that nestle in mud, wet moss, wet earth, and aquatic
plants. One species causes the cockle in wheat, appearing like a living
tuft of white wool in the blackened grains. They appear in sour paste
and in other decomposing substances, and are so tenacious of life that,
after being completely dried for months, and apparently dead, they
revive on being moistened.
_Turbellariæ._
The Turbellariæ are fresh- and salt-water animals, distinguished by
having the whole surface of their bodies covered by cilia, under which
in some species there are thread-cells containing six, eight, or a
greater number of darts. Most of the members of this tribe have
elongated flattened bodies, and move by a sort of crawling or gliding
motion over the surface of aquatic plants and animals. Some of the
smaller kinds are sufficiently transparent to allow their internal
structure to be seen by transmitted light. The mouth, which is situated
at a considerable distance from the rounded end of the body, opens into
a sort of gullet leading into the stomach, which has no other orifice,
but a great number of branching canals are prolonged from it, which
carry its contents into every part of the body. A pair of oval
nerve-centres are placed near the rounded end of the animal, whence
nerves extend to various parts of the body; and near to these there are
from two to forty rudimentary eyes according to the species, each of
which has its crystalline lens, its pigment layer, nerve bulb, and its
cornea. The power of the Planaria to reproduce portions which have been
removed is but little inferior to that of the Hydra.
_Annelids._
The Annelids are the most highly organized of all the worm tribe. They
are exceedingly numerous and varied; some are inhabitants of fresh
water, others are terrestrial, but by far the greater number and most
highly endowed are marine. They generally have a long, soft, and smooth
body, divided or marked by transverse rings into a succession of similar
segments. In many the first and last segments are alike; in others the
first segment can scarcely be called a head, though it exercises several
functions, while in the highest two orders the head is the seat of
several senses. On each side of the bodies of the Annelida there are one
or two long rows of tufted bristles or feet, which may be regarded as
the earliest form of symmetrical locomotive organs. Most of the Annelids
have ocelli or eye-specks, and in many of them the head supports soft
cylindrical tentacles, which are obviously organs of touch. These worms
are divided into four orders, the Suctorial, Terrestrial, Tubercular,
and Errantia, or Wandering Worms.[32]
The first order consists of Leeches of different kinds: their body is
long, slightly segmented, with a suctorial disc at each end. Their skin
is smooth, whitish, and translucent; beneath it are cells filled with
brown or greenish matter, and three layers of muscular fibres follow;
the first are transverse, the second cross one another diagonally so as
to form a network, and the third are longitudinal. The mouth, which
occupies the centre of the principal sucking disk, varies in form with
the genera. In the common leech it has an enlarged lip, and opens into a
short gullet leading into a capacious and singularly complicated
stomach, divided by deep constrictions into eleven compartments, the
last of which is connected with an intestinal canal, which ends in a
vent in the middle of the terminal sucker.
Within the mouth there are three crescent-shaped jaws, presenting their
convex edges towards the cavity of the mouth, beset with from seventy to
eighty teeth, formed of a highly refractive crystalline substance
resembling glass. The leech makes a vacuum with its sucker, which forces
the part to which it is applied into contact with the three-toothed
jaws, which are moved sidewise by strong muscles, and saw through the
skin and small bloodvessels below it.
The leech, like the other Annelids, has two distinct systems of
circulating liquids, one red, the other colourless. The red liquid or
blood is kept in circulation by the pulsations of a heart, or rather a
contractile vessel behind the head. It is carried away from the heart by
a pulsating canal passing along the back of the leech, and is brought
back to the heart by a similar canal extending along its ventral side.
During this course, portions of the liquid are sent off through veins to
different parts of the body. The respiratory organs of the leech are
pores arranged at regular distances on each side of the body which open
into little sacs having capillary bloodvessels distributed under the
skin through which the blood is aërated.
The colourless liquid which contains many organic molecules, occupies
the space between the alimentary canal and the inner wall of the body,
from whence it passes into canals which ramify extensively, but are not
furnished with returning passages. This liquid forms a support to the
muscles of the skin, and is kept in circulation by the motions of the
leech.
Fig. 131 shows the highly developed nervous system of the leech. From
the double lobe of the brain ten optic nerves go to the bases of ten
black eye-specks, which mark at equal distances the upper margin of the
expanded lip. A nerve-centre below the gullet supplies the lip and jaws
with strong nerves. A double longitudinal cord, united at equal
distances by twenty-one double nerve-centres, extends from a ring round
the gullet throughout the whole length of the body, supplies the
different organs with nerves, and ends near the vent in a nerve-centre,
from whence nerves radiate through the terminal sucker.
[Illustration: Fig. 131. Nervous System of Leech.]
The circulation of the blood and of the colourless liquid, as well as
the nerve system, prevail generally in the Annelids, modified by the
structure of the individual.
The leech, though greedy of blood, lives in fresh-water ponds, wet
grass, and damp places, where it never can meet with warm-blooded
animals. It probably lives on minute aquatic insects.
The common Earth-worm, which is a principal member of the second order
of Annelids, has a more important part assigned to it in the economy of
nature than its humble appearance leads us to suspect. It has a long,
soft, cylindrical body tapering to a point at both ends, divided into
numerous rings. The mouth is furnished with a short proboscis, or snout,
without teeth. A long salivary glandular mass surrounds a short wide
gullet, which leads to a digestive organ similar to a gizzard, whence a
canal is continued to the vent. The circulation of the two fluids, and
the nervous system modified at head and tail, are like those of the
leech. Four rows of minute bristles extend longitudinally along the
ventral surface of the worm, two on each side. With a low
magnifying-power they appear to be minute points regularly pushed out
and drawn in; but when more highly magnified each point is seen to
consist of two transparent glassy rods having their points bent
backwards: on these feet the worm crawls very rapidly.
While making its cylindrical burrow a slimy mucus exudes from the body
of the worm, which cements the particles of earth together and renders
the walls of the burrow perfectly smooth and slippery. When the worm
pierces the earth it stretches its snout into a fine point that it may
penetrate more easily, and when it is fixed, it draws its ringed body
towards its head by a muscular effort; and to prevent it from slipping
back again, it fixes the hooks of its posterior feet firmly into the
ground. Having thus secured a point of support it penetrates deeper into
the earth, draws up its body, fixes the hooks of the posterior feet into
the smooth surface of the burrow, and continues the same process till
the burrow is deep enough. Thus the feet are employed as points of
resistance for the exertion of muscular force. This worm swallows earth
mixed with decaying animal and vegetable matter, assimilates the
nutritive part, and casts out the refuse in the form of fine mould,
which may be seen in little heaps at the edges of their burrows. In
fact, nearly all the fine vegetable mould so precious to gardeners and
farmers has passed through the intestines of the common earth-worm.
[Illustration: Fig. 132. Foot of Naïs.]
There is a colourless little fresh-water species of the genus Naïs,
remarkable for the beauty of its bristled feet. There are two pairs on
each ring of the worm, consisting of wart-like perforated protuberances,
through which a number of microscopic bristles protrude, arranged in a
radiating pencil like a fan. They are very slender, bent at the tip, and
so transparent that they look like threads of spun glass; the worm
thrusts them out and draws them in with extreme rapidity.
A blood-red Naïs lives in burrows in the mud at the bottom of springs
and pools in immense multitudes; large tracts of the mud of the Thames
are red with a species of them; half of their bodies stretched out of
their burrows maintain a constant oscillating motion on its surface,
but, like the earth-worm, they instantly shrink into their burrows on
the least alarm. They have no respiratory organs; but their blood is
aërated through their skin, which is so transparent that, with a
microscope, the whole of the internal structure, the motions of the
liquids, and the particles they contain are distinctly visible. The
blood acts the part of internal gills, by aërating the colourless liquid
contained in a set of vascular coils surrounding the organs of
digestion.
The Tubicola are marine worms, forming the third order of Annelida,
according to the system of M. Milne-Edwards. They live in tubes, either
of a shelly calcareous substance, which forms naturally on the tenacious
mucus of their skins, or in tubes artificially constructed by themselves
of sand and particles of shell glued together. All the Tubicola can
protrude their gills and the anterior part of their bodies, and some can
leave their tube and return to it again. These worms, which form
beautiful objects for the microscope, have ringed bodies with tubular
bristled feet, and respiratory organs or gills fixed either on the head
or near it. They have an alimentary canal loosely attached to the
ventral wall of the body, and two systems of circulating liquids, one
red, the other colourless. In the Tubular Annelids the principal organs
of respiration are the contractile plumes on the head.
In the Terebella there are distinct organs for the aëration of both
liquids, which form a beautiful plume when expanded, as in fig. 133,
which shows the animal when out of its tube. What may be called the head
is fixed upon the first ring of the body. The mouth has a lip like a
funnel-shaped cup with numerous long slender tubular tentacles; and two
delicate arborescent branches or gills are fixed immediately behind the
head. The colourless liquid which occupies the space between the
alimentary canal and the ventral wall of the worm, is sent by the
contractions of the body into the slender tubular filaments round the
mouth, which are covered by cilia, whose action continually renews the
stratum of water in contact with them. The blood in its usual course
enters the capillary tubes of the arborescent gills, where it is
oxygenized, and, after being distributed to the different parts of the
body, returns to the heart and gills again.
[Illustration: Fig. 133. Terebella conchilega.—_a_, lip, surrounded by
tentacles, _b b_, all placed upon the first segment of the body, _c_;
the skin of the back, _d_, is laid open, exposing the circulatory
system; _e_, pharynx; _f_, intestine; _g_, muscles of the belly; _h_,
gland, supposed to be the liver; _i_, generative organs; _j_, feet; _k
k_, gills; _l_, heart; _m_, dorso-intestinal vessel; _n_, intestinal
vessel; _n_, venous sinus; _o o_, ventral trunk, branching into smaller
veins, _p_.]
The slender filaments which radiate from the head of the tubicular worms
are flattened, sometimes tortuous, always ciliated, and are often barred
and variegated by bright purple, green, and yellow tints, forming a rich
and gorgeous crown.
The mucus, which cements together the particles of sand and shell for
the artificial tubes of this kind of worms, is believed to be secreted
from glands in the first segment of the body; but the long slender
filaments of the head are the active agents in the structure. The
tentacles are hollow bands with strong muscular edges, which the worm
can bring together so as to form a cylinder, at any point of which it
can take up a particle of sand, or a whole row of particles, and apply
them to its glutinous body. The fibres at the free ends of the tentacles
act both as muscular and suctorial organs; for when the worm is going to
seize a particle of sand or food, the extremity of the tentacle is drawn
in by the reflux of the colourless liquid in its interior, so that a
cup-shaped cavity is formed in which the particle is secured by
atmospheric pressure, aided by the power of the circular muscular fibres
at the extremity of the tentacle.
[Illustration: Fig. 134. Pushing poles of Serpula.]
The Serpula and its allies are richly-coloured worms, living in
contorted tubes with lids, frequently seen encrusting rocks, the shells
of oysters, and other mollusca. By a peculiar mechanism of their bristly
feet they can open the lid of their tube, push out their fan of gorgeous
tentacles, pull it in again, and shut up the tube. As the protrusion of
the worm from its tube is slow, cautious, and gradual, the retreat swift
and sudden as lightning, there are two distinct sets of organs in the
feet by which these motions are performed.[33]
On the back of the worm there is a sort of shield, the sides of which
bear seven pairs of wart-like feet, which are perforated for the working
of protrusile microscopic bristles (fig. 134). Their upper parts are
double-edged, with a groove between them, and serrated with close-set
teeth. The organs of retreat are much more complicated and numerous. Mr.
Gosse has computed that there are about 1,900 blades on the seven pairs
of feet, each movable at the will of the worm, and that there are nearly
10,000 teeth hooked into the lining of the tube when it wishes to
retreat. The manner in which it comes out of its tube and retires into
it again is the same as that employed by the earth-worm.
There are twenty-four genera of the order Errantia, or wandering
sea-worms. Multitudes swarm on every coast; they have considerable
muscular strength, and are highly irritable; some are called
sea-centipedes, from the number of their feet and length of their
segmented bodies, which are slender, and vary from a few inches or less
to thirty-five or forty feet. They are generally coiled up under stones,
or wander by the slipperiness of their smooth skins through masses of
sea-weeds or shells at low tide. In most of them the rings are decidedly
marked; the first and last segments are unlike, while the rest are mere
repetitions one of another. Their locomotive organs are a pair of
perforated fleshy warts on each of their numerous segments, through
which groups of rigid, simple or barbed bristles are protruded and
retracted.
The Errant Worms have a distinct small head with a mouth, or rather an
orifice, on the upper side of it, through which a cylindrical gullet is
from time to time turned inside out, forming a kind of pear-shaped bag,
whose surface is studded with secreting glands; and its extremity, which
is perforated, is surrounded by a muscle that contracts strongly on
whatever it is applied to, and holds it firmly while the re-inversion of
the sac draws it into the body to be digested. This apparatus is unarmed
in the genera Arenicola, Phyllodoce, and others, but in the Nereis it
has one pair of strong curved horny jaws. In the Eunice there are three
toothed jaws on one side and four jaws on the other side of the gullet,
each pair having a different form, and the tiny Lombrinereis has eight
little black hooks which are seen through its pellucid tissues, snapping
like so many pairs of hooked scissors. The Errant Worms are voraciously
carnivorous, and when the gullet is turned inside out the toothed jaws
project, seize the prey, and drag it into a ciliated alimentary canal,
for there is no proper stomach in these worms. The canal is generally
straight, and terminates in a vent at the posterior end of the body.
The respiratory organs of the Errantia are external gills of great
variety of forms: they are chiefly like branching trees, or filamentary
bushes, traversed by capillary bloodvessels. They are sometimes small,
and arranged on every segment along both sides of the back; sometimes
they are large and fixed only at intervals. Like the lower Annelids,
they have two liquid systems, one red and the other colourless, and the
circulation of the blood is the same; but as the pulsations of the
vessel behind the head are too feeble to send the blood through the
labyrinth of capillary vessels in these long worms, there is a
supplementary heart, or pulsating vessel, in each segment of the worm,
which partakes in and facilitates the general circulation.
The Eunice and other very long worms may have hundreds of these centres
of propulsion, which make the circulation rapid; and it is increased by
the restlessness and activity of the worms themselves, which bring their
gills perpetually into new strata of water.
The nervous system of the Errantia consists of a double cord extending
along the ventral side of the body, and united at equal intervals by
double nerve-centres, as in fig. 131; but in the Annelids the two cords
diverge below the gullet, surround it, unite again above that tube, and
form a principal bilobed nerve-centre or brain. Each segment of the worm
is occupied by a small double nerve-centre. In some of these marine
worms there are hundreds of segments and as many nerve-centres. There
are more than a thousand of these pairs of nerve-centres on the ventral
cord of the Nemertes gigas, or Great Band Worm, which is sometimes forty
feet long and an inch broad. The head is like a snake, and the bristled
feet are jointed to enable it to move over hard surfaces.
The movements of the bristly feet of the Errantia are reflex, depending
on the nerve-centres in their segments; but they are controlled and
connected by the double cord which passes through them.
Every hair, cirrus, and tentacle on the bodies of the Errant Worms is a
living organ of feeling, shrinking at the smallest touch, but enabling
them to select their food, to move towards and retreat from objects, and
to thread their way through the most intricate labyrinths with unerring
certainty, which seems to render them independent of eyes; yet many of
them have multitudes of eyes, or rather eye-specks, according to the
genera. Some have but one eye-speck placed in the forehead; one genus
has a double row throughout their whole length, two in each segment,
while the Amphicora has two in its tail. All these eye-specks have their
crystalline lens, pigment-layer, and nerve-bulb, so that the Errant
Worms must see objects, and their motions show that they do; but we can
form no idea of the kind of vision.
Besides the variety of organs on the skin of the Errantia, some of these
worms have two rows of flat plates on their backs overlapping each other
at their edges like the scales of a fish. They are well developed in the
Aphrodita hystrix, or the Sea Mouse of fishermen, and its congeners.
That Annelid, which is an inhabitant of European coasts, is thicker and
broader than other sea-worms. The two rows of overlapping shields on its
back, and the quantity of iridescent hairs, cirri, and other appendages
covering the body, is so great as to form a kind of felt or fur like the
skin of a mouse. The members of this genus of sea-worms have no gills
properly so called; the only external sign of respiration is a
periodical elevation and depression of the shields on their backs by the
action of a complex system of muscles. The thick covering of felt on the
body of the worm below the shields becomes filled with water during
their elevation, which is ejected forcibly at the posterior end of the
body during their depression. Although the water does not penetrate the
thin skin on the back of the worm, its oxygen does, and is accumulated
in the colourless liquid in which the stomach floats; and from it the
blood, which is of a pale yellow colour, receives its oxygen. The feet
of the worm are fan-shaped groups of sharp glassy bristles enclosed
between two plates, which prevent them from hurting the animal when it
puts them out or draws them in. The Aphrodita is male and female: the
eggs escape through pores in the female, and are received in a kind of
pouch beneath the dorsal shields till hatched. The embryo is an oval
locomotive mass, with groups of cilia, and indications of an eye-speck:
after swimming about for twenty-four hours, the segments begin to be
developed.
Worms of the genus Polynoë have also two rows of shields on their backs,
but they are studded with transparent oval bodies on short stems,
supposed to be organs of touch. The filiform tentacles and antennæ that
are developed between the shields, as well as the cirri or curly
bristles of the feet, are likewise covered with similar sensitive
organs. Fig. 135 shows the foot, cirri, and bristles of a Polynoë, which
are enclosed in plates which preserve them from hurting the worm. These
glassy bristles are beautiful objects under the microscope; still more
so are the jointed feet, transparent as the purest flint glass, of the
Phyllodoce viridis, one of the most beautiful Annelids on our coasts,
where it threads its way among young mollusca like a slender green cord,
exhibiting foliaceous gills in the highest perfection.
[Illustration: Fig. 135. Foot of a Polynoë.]
In the marine Annelids the embryo, on leaving the egg, is a gelatinous
globular mass of cells furnished with strong cilia. In a few hours the
mass elongates and divides into four parts, a head, a large ciliated
segment, a smaller one without cilia, and a ciliated tail. After a time
a succession of new segments are interposed, one by one, next to the
tail segment, and the corresponding internal organs of each are
developed till the worm arrives at its adult state. In many Annelids the
embryo is highly developed within the parent; that of the Eunice has
from 100 to 120 segments before it leaves her; and in the Nereis
diversicolor the young, covered with cilia, come out by hundreds at an
orifice in the side of the mother.
Many of the marine Annelids are luminous; electric scintillations are
given out during the act of nervous contraction, which are increased in
brilliancy and rapidity by irritation.
According to Professeur Quatrefages, the Annelida Errantia and Tubicola
have no zoological regions characterized by one or more special types
like the other classes of animals; they have representatives in all
seas. But it is exactly the contrary with regard to species. The number
of species common to any two seas, or the shores of two continents, is
very small; there is not a single species common to the Atlantic coasts
of France and the Mediterranean. The sea-worms are not affected by
climate, but they are said to be more abundant on granitic and schistose
coasts than on the calcareous.[34]
With regard to fossil remains, worm-tracks are seen in the Forest
marble, long calcareous tubes occur in the Upper Silurian and
Carboniferous strata, and in all the later formations tubercular
Annelids abound, especially of the genera Serpula, Spirorbis, and
Vermilia.[35]
_Tardigrada._
The Tardigrades are slow creeping animalcules, which seem to form a link
between the Worms and the Rotifers, though they are more nearly allied
to the former in having a vermiform body divided transversely into five
segments, the first of which is the head, and each of the others has a
pair of little fleshy protuberances furnished with four curled hooks.
They resemble the Rotifers in their jaws, in their general grade of
organization, and in the extreme length of time they can remain dried up
without loss of life. When in the dried state they can be heated to a
temperature of 250° Fahr. without the destruction of life, although when
in full activity they cannot endure a temperature of more than from 112°
to 115° Fahr. When alive the transparency of their skin is such as to
show a complicated muscular system, the fibre of which is smooth; and as
no respiratory organs have yet been found, their respiration must be
cutaneous. These animalcules have no nerve-centre in the head, but they
have one in each segment of the body; and they are furnished with a
suctorial mouth at the end of a retractile proboscis, on each side of
which are two tooth-like styles, the rudiments of lateral jaws. The
structure of these creatures is microscopic.
_Rotifera._
Although the Rotifera are microscopic objects, their organization is
higher than that of the Annelida in some respects. They are minute
animalcules, which appear in vegetable infusions and in sea-water, but
by far the greater number are found in fresh-water pools long exposed to
the air: occasionally they appear in enormous numbers in cisterns which
have neither shelter nor cover; a few can live in moist earth, and
sometimes individuals are seen in the large cells of the Sphagnum or
Bog-Moss.
The bodies of the Rotifers have no cilia; they are perfectly
transparent, elongated, or vermiform, but not segmented; they have two
coats, both of which in some genera are so soft and flexible that the
animal can assume a variety of forms; while in others the external coat
is a gelatinous horny cylindrical shell or tunic enclosing the whole
body except the two extremities, which the animal can protrude or draw
in. The soft kind can crawl over solid surfaces by the alternate
contraction and extension of their bodies like a worm, and the stiff
Rotifers are capable of doing the same by the contractility of their
head and tail. All can swim by means of cilia or lobes at their head.
The greater number possess means of attaching themselves to objects by
the posterior end of their bodies and of removing to another place.
The wheel-like organs from which the class has its name, are most
characteristic in the common Rotifer (fig. 137), where they consist of
two disk-like lobes projecting from the body whose margins are fringed
with long cilia. It is the uninterrupted succession of strokes given by
these cilia, passing consecutively like waves along the lobes, and
apparently returning into themselves, which gives the impression of two
wheels in rapid rotation round their axes.
The Brachionus pala (fig. 136) affords another instance of the
two-wheeled Rotifers. Though of unusually large dimensions in its class,
it is just visible to the naked eye as a brilliant particle of diamond
when moving in a glass of water. Its transparent horny tunic, when
viewed in front with a microscope, is a cup of elegant form, bulging at
the sides. One side of the rim is furnished with four spines, of which
the middle pair are slender and sharp as needles, with a deep cleft
between them; the other side of the rim is undulated but not toothed,
and the bottom of the cup ends in two broad blunt points.
Between the terminal blunt points there is a round opening for the
protrusion of the foot of the animal. The tunic is of glassy
transparency, so that every organ and function of the animal can be
traced with perfect distinctness.
[Illustration: Fig. 136. Brachionus pala, with three eggs attached to
its foot.]
The foot of the animal is long, rough and wrinkled, not unlike the
flexible trunk of an elephant. It can be lengthened, shortened, drawn
within, or pushed out of the tunic in an instant. It terminates in two
short conical fingers or toes, which can be widely separated or brought
into contact. By means of these, the Brachion has the power of mooring
itself even to the smooth surface of glass so firmly, that it can
stretch itself in all directions, shaking itself to and fro with sudden
violence without letting go its hold. The Rotifers usually fix
themselves before they set their wheels in motion in search of food.
From the anterior rim of the shelly cup, the Brachion protrudes a waved
outline of limpid flesh which, as soon as it rises above the level of
the sharp-pointed spines, spreads out into three broad flattish muscular
lobes. On the edges of the middle one there are very strong cilia like
stiff bristles, which do not vibrate, but are either erect or converge
to a point, whereas the edges of the other two lobes are thickly fringed
with long stout cilia, which, by striking the water in perpetual rapid
succession, each cilium bending and rising again, produce the appearance
of two circles of dark spots in rapid horizontal rotation, like wheels
on their axis. It is merely an optical deception, for both the animal
and its lobes may be at rest. The vibrations of the cilia can be
instantaneously arrested, and the whole apparatus drawn out of sight,
and as instantaneously protruded and set in motion.
In the flesh, on the ventral side of the Brachion, there is a deep
cleft, the edges of which as well as the whole interior of a tube of
which it is the orifice, are thickly covered with vibratile cilia. This
tube leads to a mouth with powerful jaws of unwonted structure, which is
so deeply sunk in the tissues of the body, that it never comes into
contact with the water. It opens into a gullet leading to a stomach,
intestine, and vent, at the posterior end of the body.
The vibrations of the cilia on the lobes of the animal’s head form two
circular currents in the water, like whirlpools, which draw all floating
particles into their vortices, and the streams from the two whirlpools
uniting into one current, flow off horizontally and pass immediately
over the slit on the ventral side of the animal. Some of the floating
particles are arrested by the cilia on the edges of the slit, and are
drawn into the sunken mouth by the vibrations of the cilia in the tube.
The edges of the slit act like lips, and seem to possess the sense of
taste, or of some modification of touch, which enables them to select
from the particles presented to them, such as are fit for food; these
are admitted into the mouth, where they are bruised by the powerful
jaws. The mouth or masticating apparatus is the most extraordinary and
complex part of this animal. It consists of two horny toothed jaws,
acting like hammers upon an anvil. The two hammers, which approach each
other from the dorsal sides of the body, are each formed of two parts
united by a hinge; the first parts correspond to the handles; the second
parts, which are bent at right angles to the first, resemble hands with
five or six finger-shaped teeth united by a thin membrane. The teeth are
parallel to one another when they meet on the anvil, and are seen
through the transparent mass tearing the food into fragments. Some of
the Rotifers resemble the Errant Annelids in being able to turn this
complicated machine inside-out through the ciliated tube and slit, so as
to bring it into contact with the water. When the food has been
masticated it is sent into the stomach, where it is digested. The whole
of this process is seen through the transparent and colourless body of
the Brachion, because its favourite food is the Syncryn velox, a minute
bright green plant, which from its active motions was at one time
believed to be an animal.
The Brachion has four longitudinal muscular bands transversely striated,
which move the ciliated lobes of the head, push them out and draw them
in. From these muscular threads are sent to the different parts of the
body, to the mouth especially, two strong bands, which bend and unbend
the joints of the hammer-like jaws. The vigorous motions of the long
serpentine foot and the firm hold of its anchors are owing to muscular
bands fixed high up on the interior wall of the body, which extend
throughout the whole length of the flexible organ. As long as the
Brachion is fixed, the vibrations of the cilia on its lobes only produce
whirlpools in the water, but the moment that it lets go its hold, these
vibrations, in consequence of the reaction of the water, give the animal
both a smooth progressive motion and a rotation round its axis.
Minute as the Brachionus pala is, it has several organs of sense. A
sparkling, ruby-coloured, square eye-speck with a crystalline lens and
crimson pigment layer is placed on a wart-like prominence on its back,
and this prominence Mr. Gosse believes to be the brain of the animal. In
the cleft between the spines and close to the eye-speck are two tubes,
one within the other. The innermost tube, which can be protruded and
withdrawn, has a bunch of bristles at its extremity that have the
sensibility of antennæ. Nerves from the brain pass into these, to the
various organs of the body, and to the lobes on the head.
The Brachion has no propelling vessel or heart to maintain the
circulation of its liquids, but, like the Annelids, a colourless liquid
occupies the general cavity between the alimentary canal and the
internal wall of the body. It is believed to be connected with
nutrition, and is furnished with oxygen by a complicated organism, and
is kept in motion by the vibrations of long cilia. The determination of
the whole structure and motions of a creature barely visible to the
naked eye, is a wonderful instance of microscopic research, and of the
perfection of the mechanism exhibited in the most minute objects of
creation.
[Illustration: Fig. 137. Common Rotifer:—_a_, mouth; _b_, eye-spots;
_c_, wheels; _d_, probably antenna; _e_, jaws and teeth; _f_, alimentary
canal; _g_, glandular mass enclosing it; _h_, longitudinal muscles; _i_,
tubes of water-vascular system; _k_, young animal; _l_, cloaca.]
Fig. 137 represents the common Rotifer when its wheels are expanded and
when they are retracted. The body is slender and flexible, it is
stretched out by longitudinal muscles, and its girth is diminished by
circular ones. The internal structure is similar to that of the
Brachion, but there is a prominence or head between the wheels on which
there are two crimson eye-specks, and the foot terminates in three
concentric movable tubes that can be protruded and drawn in like the
tubes of a telescope; each has a pair of claspers to enable the Rotifer
to fix itself to any object.
The Rotifers are male and female, but, like the greater number of
Infusoria, the males are only produced at intervals. The female Rotifers
have their perfect form when they leave the egg: they even come out of
the egg while it is attached to the tail of the mother, as in the
Brachionus pala (fig. 136). The males, when hatched, have neither spines
nor mouth, yet, during their short lives, their motions are very fleet
on account of the vibrations of long cilia round their front.
Some Rotifers are remarkably fertile. Professor Ehrenberg estimated
that, in the course of twenty-four days, the offspring of a single
individual of the genus Hydatina might amount to seventeen millions.
Female eggs laid in autumn are collected in heaps and covered with a
gelatinous substance, which protects them from the cold in winter,
though the Rotifers themselves are sufficiently protected by their great
tenacity of life. They revive after being frozen; they may be dried for
an unlimited time, but, as soon as they meet with warmth, moisture, and
food, they resume their vitality.