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W. T. (William Thompson) Sedgwick.

An introduction to general biology online

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a one-celled body.

The protoplasTii (cytoplasm) consists of a clear basis, and (in
the case of the entoplasm) of innumerable granules extremely
diverse in form and size, and frequently differing in character
in different individuals. Often thev are in the form of rhom-
boidal crystalline bodies; in other cases they are rounded or
irregular. Their precise chemical composition is uncertain, but
they are probably comj^lex organic compounds, a product of
metabolism and serving as reserve food-matter.*

Vacuoles. The protoplasm often contains rounded vacuoles
of which the three following kinds may be distinguished :

{ft) Water -vacuoles {w.v, Figs. 84, 85), filled with water,
lying in the entoplasm and carried along in its currents.

(h) jFood -vacuoles {f.v)^ also lying in the entoplasm, con-
taining the solid food-matters that have been ingulfed. Within
them digestion takes place. When this process is completed
they approach the exterior — usually at some point near the
posterior end — the outer wall breaks through, and the innutri-
tions remnants are cast out, the ectoplasm closing up the breach
immediately afterwards. Thus Amoeba has no mouth, ali-
mentary canal, or anus, but the general mass of protoplasm
plays the role of all three.

{c) Contractile vacuole (c.v). Usually single, sometimes
double, lying near the posterior end, and filled with liquid.
This is sharply distinguished from the other vacuoles by its
rhythmical pulsation, expanding (diastole) and contracting {sys-
tole) at regular intervals. During the diastole the vacuole slowly
fills with liquid which drains into it from the surrounding j)roto-
plasm. At the systole, which is very sudden, tliis liquid is forci-
bly expelled to the exterior through an opening that breaks

* In some species of Amoeba the entoplasm may also contain innumerable
grains of sand taken in from the exterior, but this is not the case in A. Proteus.



PHYSIOLOGY OF AMCEBA. 163

through the ectoplasm, and immediately afterwards disappears.
The contractile vacuole is almost certainly to be regarded as a
simple kind of excretory apparatus, the water which collects in
it containing in solution various products of destructive metabo-
lism which are thus passed out of the body.*

Reproduction. However abundant the food-supply Amceha
never grows beyond a certain maximum limit. x\fter this limit
has been attained the animal sooner or later divides by '^ fission'*''
into two smaller AmoebcB (Fig. 85, A). Thus the existence of
an individual Amoeha is normally terminated, not by death, but
by resolution into two new individuals. This process is the
simplest possible form of agamogenesis, and Amoeba is not known
to multiply in any other way.f The fission of Amoeha is a
process essentially of the same nature as the division of ordinary
tissue-cells, a division of the nucleus preceding that of the
cytoplasm. Whether the division of the nucleus is of tlie indi-
rect type (i. e. , passes through the phenomena of karyokinesis) is
not known by direct observation, but there is some reason to be-
lieve that it is so. In any case the successive fissions of Amoeba
are directly comparable with the successive cleavages of the e^^
of a metazoon (p. 25). Tlie progeny of the Amoeba^ however,
separate and form independent individuals, while those of the egg-
cell remain intimately associated to form a single multi-cellular
individual. Morphologically, therefore, a metazoon is comparable
not with a single Amoeba^ but mth a multitude of Amoebo^.

Physiology. The possible simplicity of animal structure is
well shown in Atnoeba^ which is morphologically an animal re-
duced to its lowest terms. Its physiological ojierations are cor-
respondingly primitive and rudimentary ; and by an analysis of
them we may discover what is essential and fundamental in the
physiology of animals in general. A survey of the various activ-
ities of Amoeba shows that these may all be reduced toaiewfunda-
inental ])liysiological j^y^oioerties of tlie protoplasm, ;[: as follows:

* It may be recalled that the cavity of the nephridium in the earthworm is
intra-cellular, like a vacuole (p. 60).

f It has been asserted that Amceba conjugates and also that it multiplies by
endogenous division ; but the evidence on both these points is inconclusive,

X It is hardly necessary to remark that in common with all English-speak-
ing biologists we are indebted to Foster for the first comprehensive elaboration
of the " fundamental physiological properties " as exhibited by A7naba.



164 UNICELLULAR ANIMALS.

(1) Contractility^ by means of which motion is effected.
This appears most clearly when the animal is stimulated by a
sudden jar, or by an electric shock, which causes the body to
contract into a ball. This property, precisely like the contraction
of a muscle (p. 27), is the result of a molecular rearrangement,
accompanied by chemical changes, which causes a change of
form in the mass without altering its bulk. The action of the
contractile vacuole is due to the contractility of the surrounding
protoplasm ; and in like manner the currents which cause the
protrusion and withdrawal of pseudopods, and so the locomotion
of the animal as a whole, are produced by localized contractions
of the i^eriplieral layer of protoplasm which drive onwards the
more fluid central parts.

(2) IrritaMlity {including Co-ordination)^ or the power to
be afliected by, and to respond to, changes or ' ' stimuli ' ' acting
upon or within the protoplasm. The change of shape following
the application of an electric shock is actually effected by con-
tractility, but the power to be affected by the shock and to arouse
contractility, is irritability. To this property the animal owes its
power of performing adaptive actions in response to changes in
the environment, and also its power to co-ordinate the various
actions of its own body. To illustrate : It is a remarkable fact
that Amoeba is able to discriminate between nutritious and innu-
tritious matters, ingulfing the former, but rejecting the latter.
Physiologically this discrimination is a difference of response to
different stimuli — hence a phenomenon of irritability. Again,
the various actions (movements, etc.) of Amceba^ despite their
apparently vague character, are co-ordinated to form a definite
whole ; and co-ordination may be regarded as a phenomenon of
irritability, changes in one part serving as stimuli to other parts
and being brought into orderly relation with them. The property
of irritability lies at the base of all nervous activity in higher
forms (cf. p. 67) and is concerned in many other actions.

(3) Metdbolism^ the most fundamental of all vital actions,
since it lies at the root of all, is the power of waste and repair —
the destructive chemical changes in protoplasm {katabolism)
whereby energy is set free, and the constructive actions (anabo-
lisrri) through which new protoplasm is built and potential
energy is stored (cf. p. 33). There is every reason to believe



PHYSIOLOGICAL PROPERTIES OF AM(EBA 165

that the metabolic phenomena of Amoeba are, broadly speaking,
similar to those of higher animals. The katabulic changes are in
the long run processes of oxidation, and although their products
have not yet been definitely ascertained in Amoiba^ there can be
no doubt that they consist mainly of carbon dioxide, water, and
some form of nitrogenous matter (urea or a related substance).
Most of these waste matters are believed to be j:)assed out {se-
cretion^ excretion) by means of the contractile vacuole, l)ut prob-
ably carbon dioxide leaves the body by diffusion through the
general surface {respiration in part).

The materials for the constructive process {anaholisin) are
derived from organic food-matters — bodies or fragments of })lants
and animals taken as food in the process of alimentation^ and
absolution from the water and the inorganic salts diss(jlved in
it, and from the free oxygen that enters by diffusion through
the general surface {respiration in part). Proteid matter is an
indispensable constituent of the food, and Amaiba is therefore
an animal.

Alimentation, absorption, secretion, digestion, and circula-
tion, all of which are only the prelude to metabolism, but which
in the higher animals are assigned to different organs, tissues,
and cells, are here performed by one and the same cell. The
capture of solid food here requires its entrance into the cell ;
and the fact that proteids cannot be absorbed by diffusion neces-
sitates intracellular digestion which in turn necessitates cellular
defaecation. It will be observed that while there is no localized
or permanent mouth or anus, the whole surface of the cell is
potentially mouth or anus. In short, the protoplasm here ex-
hibits not the physiological division of labor, but its absence.

(4) Growth and Reproduction. Logically there is in the
case of Amoeba no good ground for a distinction between these
processes and metabolism ; for reproduction is directly or indi-
rectly an effect of growth, and growth is simply an excess of
anabolism over katabolism. Practically, however, the distinc-
tion is necessary; for the tendency of living things to run in
cycles of growth and reproduction is one of their most obvious
and characteristic features.

Here, as in all protoplasmic structures, growth takes })lace
throughout the mass, by intussusception (p. 4), not by the ad-



166 UNICELLULAR ANIMALS.

ditions of superficial layers, as in the case with growth by accre-
tion (inorganic bodies, e.g., crystals). Under favorable condi-
tion of nutrition this process exceeds the destructive process so
that the body increases in size up to a limit, at which fission
takes place. AVhat determines this limit is unknown, but the
cause is j)erhaps in some way connected with the geometrical
principle that the volume of the cell increases as the cube of its
diameter, whereas the surface, by wliich it absorbs nutriment,
and otherwise comes into relation with the outside world, in-
creases only as the square of the diameter. 'No great increase
in size, therefore, is possible without destroying the normal equi-
librium of the cell and hence the periodic reduction of size by
division. This principle is, however, too general to be of much
value. Different species of Amceha differ in size-limit, and the
immediate cause lies in some subtle relation between organism
and environment that cannot at present be made out. It is not
known whether or not the Amoeba ever dies of old age.

These "fundamental physiological properties" of proto-
plasm lie at the basis of all physiology, and will be found ap-
plicable to all forms of life whether vegetal or animal.

Related Forms. Amoeba is a representative of a very extensive class of
Protozoa known as Rhizojjoda^ all characterized by the power to form
pseudopodia, and agreeing with Amoeba in many other respects. One of
the commonest fresh- w^ater forms is the genus A7'ceUa (Fig. 86, C), which
even in the active phase is surrounded by a brown horny membrane
("shell") perforated by a large rounded opening through w^hich pseudo-
podia are protruded. DiMugia (Fig. 86, B), also a common fresh-water
form, builds about itself a beautiful vase-shaped or retort-shaped shell
composed of sand-grains, or even, in some cases, of diatom-shells. In
ActinopJirys^ or the "sun-animalcule" (Fig. 86, A), the pseudopodia are
stiff needle-shaped processes radiating in every direction.

Among the marine forms two groups (orders) are of especial interest
and importance ; viz., the Foraminifera, which secrete a calcareous shell
perforated by numerous pores, and the Radiolaria^ which have a siliceous
shell. Many of these forms float at the surface of the water, and their
cast-off shells have in former times accumulated at the bottom in such
enormous quantities as to form beds of chalk in the case of Foraminifera,
while the remains of Radiolaria have made important contributions to the
formation of siliceous rocks.



FRESU-WATER RUIZOPODS.



167



\






\



V




€^






























1%I



Fig. 86.— Group of cortmon fresh-water Rhizopods (after Loidy). A, Actinnjihriix
sol, the "sun-animalcule," filled with vacuoles and containin.LT three food-bodies
(zoospores of an alga) ; a fourth is just being ingulfed. The nucleus is i^ot seen.

B, Difflugia urceolata, with shell built of sand-grains and pseudopodia far ex-
tended.

C, Arcella mitrata, a transparent individual showing the protoplasmic body sus-
pended within the shell ; several vacuoles are shown, but no nucleus.

(Highly magnified.)



CHAPTEE XIII.

UNICELLULAR ANIMALS (PROTOZOA) {Continued).

B. Infusoria.

{Paramoecium, Vorticella, etc.)

Infusoria are minute unicellular animals found like Amoeba
in stagnant water or in organic infusions (see p. 201) (hence
"Infusoria"). In the leading features of their organization
they are closely similar to Amoeba and its allies, from which
they differ, however, in having a much higher degree of differ-
entiation, in mo^dng by means of cilia instead of j)seudopodia,
and in showing the first indication of gamogenesis (amphimixis).
Paramoeciimi (the slipper-animalcule) is an actively free-
swimming form often found in multitudes in hay-infusion or
water containing the decomposing remains of Nitella and other
water-plants. Yorticella (the "bell-animalcule ") is commonly
attached by a slender stalk to duck-weed (Lemnoi) and other
water-plants, or to other submerged objects ; at other times it
breaks loose from the stalk and swims for a while actively about.
The two forms are constructed upon essentially the same plan,
but Yorticella shows in some respects a much higher degree of
differentiation.

Paramoecium. — The slipper-shaped body (Fig. 87) is covered
with cilia by means of which the animal rapidly swims about.
Morphologically the body is a single cell, having the same gen-
eral composition as in Amoeba^ but possessing in addition a deli-
cate surrounding membrane ("cuticle") or cell-wall. The
differentiation of the protoplasm into ectoplasm and entoplasm
is very sharply marked, and the former contains numerous
peculiar rod-like bodies {trichoGysts) from which long threads may
be thrown out. Their function is probably that of offence and
protection. As in Amoeba the protoplasm contains water-vacu-

oles (w.v) and food-vacuoles (f.v) (both of which are carried

168



TUE SLIPPER-ANIMALCULE.



169



?7^tO




cv*



-mm^^- ■ "



Fig. 87.— Param(xcium caudatum. A, from the left side, showing: the anal spot : 7?,
from the ventral side, showing the vestibule en f(xcc; arrows inside the body in-
dicate the direction of protoplasmic currents, those outside the direction of
water-currents caused by the cilia.
an, anal spot; c.r, contractile vacuoles; /c, food-vacuoles ; ic.r, water v;icuoles; m.
mouth; mnc, macronucleus; »?iiV', micronucleus; tt', oesophagus : i', vestibule. Thi>
anterior end is directed upwards.



170 UNICELLULAR ANIMALS.

about by currents in the entoplasm), and two very large contrac-
tile vacuoles (c.v) occupying a constant position, one near either
end of the body. The nucleus (as in Infusoria generally) is
differentiated into two distinct parts, viz. , a large oval macro-
nucleus {mac.) and a much smaller spherical micromccleus (mic.)
(double in some species) lying close beside it.

Unlike Aramha^ ParamrKjecium possesses a distinct mouth (m)
and oesophagus {<£) which open to the exterior through an oblique
funnel-shaped depression known as the vestibule (v) situated at
one side of the body. Minute floating food-particles are drawn
by the cilia into the mouth and accumulate in a ciliary vortex at
the bottom of the oesophagus. From time to time a bolus or
food -mass is thence passed bodily into the substance of the en-
toplasm, forming a food -vacuole within w^hich digestion takes
place. The indigestible remnants are Anally passed out not
through a permanent opening or anus, but by breaking through
the protoplasm at a definite point, hence known as the anal
spot, which is situated near the hinder end (Fig. 87). The
contractile vacuoles of Paramoecium are especially favorable for
study, showing at the moment of contraction, or just before it,
a pronounced star-shape, with long canals running out into the
protoplasm. Through these liquid is supposed to flow into the
vacuole.

Like Amoeba, Paramoecium occurs both in an active and in
an encysted state. In the former state it multiplies by trans-
verse fission, division of both macronucleus and m^icronucleus
preceding or accompanying that of the protoplasmic body (Fig.
88, A). Under favorable conditions division may take place once
in twenty-four hours, or even oftener. This process, which is a
typical case of agamogenesis, may be repeated again and again
throughout a long period. But it appears from the celebrated
researches of Maupas that even under the most favorable con-
ditions of food and temperature the process has a limit (in the
case of Stylonichia, a form related to Paramoeciuim, this limit
is reached after about 300 successive fissions). As this lunit is
approached the animals become dwarfed, show various signs of
degeneracy, and finally become incapable of taking food. The
l^ace grows old and dies.

In nature, however, this limit is probably seldom if ever



CONJUGATION OF PAliAM(ECIUM.



171



readied, and tlie degenerative tendency seems to be checked by
a process known as conjugation. In this j^rocess two individuals
place themselves side by side, partially fuse together, and remain
thus united for several hours (Figs. SS, B, C). During this
union an exchange of nuclear material is effected, after which
the annuals separate, both macro7tucleus and micronucleus now




cir



cir



A B

Fig. 88.—^. Fission of Paramoecium. (From a preparation by G. N. Calkins), mac,
macronucleus ; mic, micronucleus ; m, mouth.

B. First stage of conjugation. The animals are applied by their ventral sur-
faces; the only change thus far is the enlargement of the micronuclei.

C. Conjugation at the moment of exchange of the micronuclei (less magnified).
The macronuclei are degenerating. Each individual contains two micronuclei
(now spindle-shaped), one of which remains in the body, while the other crosses
over to fuse with the fixed micronucleus of the other individual (After Maupas.)

consisting of mixed material derived equally from both individ-
uals. Separation of the two animals is quickly followed by
fission in each.

In each individual the macronucleus breaks up and disappears. The
micronucleus of each divides twice, and of the four bodies thus produced
three disappear. The fourth divides again into two, one of which remains
in the body, while the other crosses over and fuses with one of the micro
nuclei of the other individual, after which the animals separate. This
process being reciprocal, each individual now contains a micronucleus con-



172



UNICELLULAR ANIMALS.



taining an equal amount of material from each individual. This micro-
nucleus now divides twice and gives rise to four bodies, two of which be-
come macronuclei and two micronuclei. Fission next occurs, and is there-
after continued in the usual manner.

This is a process clearly analogous to the union of the germ-
cells of higher animals. It cannot, however, be called gamo-
genesis or even reproduction ; it is only comparable with one of
the elements of gamogenesis. In the metazoon a fusion of two




Fig. 89.— Group of Vorticellce, in various attitudes, attached to the surface of a

water-plant.

cells (fertilization) is followed by a long series of cell-divisions
(cleavage of the ovum), the resulting cells being associated to
form one new individual. In the Infusoria temporary fusion
(conjugation) is likewise followed by a series of cell-divisions,
but the cells become entirely separate, each being an individual.
Vorticella agrees with Paj^mnmcium in general structure, but
differs in many interesting details, most of which are the expres-



THE BELL-ANIMALCULE.



173




mac



Fig. 90.— a single head of Vorticdla., highly magnified. c.r, contractile axis of the
stalk; c, cuticle; c.i\ contractile vacuole; d, disk; fc, ectoplasm; t/j, entoplasm;
ep, epistome ; /.i% f ood-vacuole ; m, mouth; »mc, macronucleus ; mic, micronu-
cleus ; 0?, oesophagus; p, peristome ; r, vestibule ; ir.r, water-vacuole-s ; x\ point at
which epistome and peristome meet at one end of the vestibule.



174 UNICELLULAR ANIMALS.

sion of higher differentiation. The body is pear-shaped or coni-
cal, attached at its apex by a long slender stalk. The latter
consists of a slender contractile axial filatnent^ by means of
which the stalk may be thrown into a spiral and the body drawn
down, and an elastic sheath (continuous with the general cuticle)
by which the stalk is straightened (Fig. 90). The cilia are con-
fined to a thickened rim, the peristorne (^:>), surrounding the
base of the cone, which may be termed the dislc. At one side
the disk is raised, forming a projecting angle covered with cilia,
and known as the epistome (ej)). At the same side the peristome
dips downw«ards, leaving a space between it and the ej)istome.
This space is the vestibule (v), and into it the mouth opens. In
it likewise is situated an anal spot like that of I^aramcechmi.
The cilia produce a powerful vortex centering in the mouth, by
means of which food is secured. The macronucleus {inac) is
long, slender, and horseshoe-shaped ; the small spherical micro-
nucleus (mic) lies near its middle portion. There is usually
but one contractile vacuole.

Yorticella multiplies by fission, division of the protoplasm
being accompanied by that of the macronucleus and micronu-
cleus (Fig. 91). The plane of fission is vertical (thus dividing
the peristome into halves), but extends only through the main
body, leaving the stalk undivided. At the close of the process,
therefore, the stalk bears two heads. One of these remains
attached to the original stalk, while the other folds in its peri-
stome, acquires a second belt of cilia around its middle (Fig. 91),
breaks loose from the stem, and s\dms actively about as the so-
called " motile form." Ultimately it attaches itself by the base,
loses its second belt of cilia, develops a stalk, and assumes the
ordinary form. By this process dispersal of the species is en-
sured. Under unfavorable conditions similar motile forms are
often produced wdtliout previous fission, the head simply acquir-
ing a second belt of cilia, dropping off, and swimming away to
seek more favorable surroundings. Yorticella may become en-
cysted, losing its peristome and mouth, becoming rounded in
form, acquiring a thick membrane, and having no stalk. In
this state it is said sometimes to multiply by endogenous dimsion^
breaking up into a considerable number of minute rounded
bodies (spores) each of which contains a fragment of the



CONJUGATION OF VORTICELLA.



175



nucleus. These are "finally liberated by the bursting of the
membrane, acquire a ciliated belt, and after swiuuning for a
time become attached, lose the ciliated belt, and develop a stalk
and peristome.

Yorticella goes through a process of conjugation which lias
some interesting peculiarities. (1) Conjugation always takes
place between a large attached individual (the macrogamete) and
a much smaller free-swimming individual (the nvicrogainete)



c.v






mtc



Fig. 91.— Fission and conjugation of VorticcWa. A. Early stage of fission, showing-
division of micronucleus {mic) and macronucleus {mac) ; p, peristome. (After
Biitschli. )
B, C, -D. Successive stages of fission ; in J3 and C the nuclei have completely di-
vided and fission of the cell-body is in progress; c.r, contractile vacuoles. In
JD fission is complete; the right-hand individual has acquired a belt of loco-
motor cilia at r, and is ready to swim away.
JE. Conjugation of a fixed macrogamete {ma) with a free-swimming microgamete
{imX\\ p, peristome, cp, epistome. (After Greeflf.)

(Fig. 91, K). The microgamete is formed either by the unequal
fission of an ordinary individual, the smaller moiety being set
free, or by tw'O or more rapidly succeeding fissions of an ordinary
individual. (2) Conjugation is permanent and complete, the
body of the microgamete being wholly absorbed into that of the



176 UNICELLULAR ANIMALS.

macrogamete. Witliin the body of the latter, after complicated
changes, the nuclei fuse together, and this is followed by fission.
The analogy of conjugation to the fertilization of the Qgg is here


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Online LibraryW. T. (William Thompson) SedgwickAn introduction to general biology → online text (page 15 of 20)