PROTOZÓA (πρῶτος, FIRST, ξῶον, ANIMAL).
The members of this subkingdom are the lowest in the scale of animal
organization, their bodies consisting of a soft gelatinous and
structureless mass, which has a remarkable tendency to form little
cavities or vacuoles in its substance, and is called _sar´code_ (σἀρξ,
flesh). They exhibit no organs, unless the cilia and certain variable
processes formed of the common substance of the body, and which form
their agents of locomotion, be considered as such,--this substance
exercising the combined functions of motion, sensation, and secretion,
for which separate organs exist in the higher animals.
RHIZOP´ODA (ῥἱξα, root, ποῦς, foot).--The animals belonging to this
class consist of the structureless colourless substance to which
reference has been made as sarcode, and they exhibit no organs. The
sarcodic body is slowly contractile, and portions of it can be protruded
at will in the form of irregular root-like processes, acting both as
legs for locomotion and as tentacles by which the animal grasps its
prey, which is then forced into the substance of the body, where it
becomes surrounded by the surface, and a cavity is formed, within which
it is digested.
_Amœ´ba dif´fluens_ (Pl. XI. fig. 10) is common in water in which
portions of plants have been kept for some time. When first placed on
the slide, the body appears as a minute, transparent, rounded mass of
jelly; but if observed for some time, it will be seen slowly to
protrude its root-like processes; and foreign bodies, as Diatomaceæ or
other minute Algæ, will often be found imbedded in its substance.
_Arcel´la vulgáris_ (Pl. XI. fig. 6) is found among _Confervæ_ in ponds
and ditch-water. It is contained in a hemispherical shell or carapace,
from the round orifice of which the lobed processes are protruded. The
shell is covered with minute pits.
_Arcel´la aculeáta_ (Pl. XI. fig. 7) has the convex shell furnished with
spines; fig. 8 represents the animal with its processes extended; while
_Arcella dentáta_ (Pl. XI. fig. 9) exhibits an angular or somewhat
toothed membranous shell. Both the latter species are met with in the
same localities as the first.
_Actínophrys sol_ (Pl. XI. fig. 12) is a very beautiful and excessively
delicate Rhizopod. The body is spherical, and covered with very delicate
and slender cilia-like processes. Its movements are exceedingly slow,
and can only be observed by prolonged watching. The body appears to be
reticulated, from the presence of numerous vacuoles.
Two large groups of genera and species of Rhizopoda, the animal bodies
possessing the above general characters, mostly with very slender
processes, exist, in one of which (the FORAMINIFERA) they are contained
in calcareous shells, often of elegant forms; while in the other (the
POLYCYSTINA) the shells are siliceous or composed of flint, both kinds
of shells being perforated with holes. These shells, which occur in the
fossil state in enormous numbers, sometimes forming mountain-masses, are
extremely beautiful objects for the microscope.
SPON´GIÆ.--This class contains the Sponges, almost all of which are
marine and foreign, and therefore not likely to come under observation
in the perfect state. The substance commonly called sponge is the horny
skeleton of the animal, consisting usually of rounded fibres (Pl. XI.
fig. 13), irregularly netted and interlacing. The surface of a sponge
exhibits minute pores and larger pouting orifices; the former of which
admit currents of water, to be discharged at the latter, both being the
mouths of continuous channels. The surfaces of the channels are lined
with sarcodic matter, which takes the form of ciliated amœbiform bodies,
by which the currents of liquid are produced.
The horny fibres of sponges are strengthened by little siliceous or
flinty bodies of various forms (Pl. XI. fig. 13 _a_, _b_, _c_), which
are imbedded in the substance of the fibres or attached to their
surface, and form very curious microscopic objects. They are called
_spic´ula_ (_spiculum_, a dart), being often of a pointed form. In some
sponges they are calcareous.
INFUSOR´IA.--The animals contained in this class are usually very
minute, being rarely even perceptible to the naked eye, except when
existing in very large numbers, so as to render the water milky, green,
or red. They are found in all kinds of water, but especially in stagnant
pools and in decomposing solutions or infusions of vegetable matters.
The true structure of their bodies is a matter of doubt, some authors
having considered them as being highly organized, while others have
regarded them as consisting of simple cells; and whether they are
correctly referred to the Protozoa must remain at present a matter of
doubt. The body is of various forms, as represented in Plate XI. figs.
16-40. In some of them it consists of a simple sarcodic mass, evidently
without any outer skin, as shown by its ready adhesion and laceration on
accidental contact with foreign bodies; while in others the surface is
regularly dotted with little depressions, or with nodules, so as to
resemble a definitely organized structure.
The most striking character of the Infusoria is the presence of
vibratile cilia, which are variously arranged; in some entirely
covering the body, irregularly or in regular rows, in others being
situated at definite parts only. By the action of the cilia they are
enabled to swim freely in the water, also to obtain their food, which
consists of minute Algæ or fragments of animal matter. In many of them
there is a special row or set of cilia, which, by the currents it
produces, urges the particles of food suspended in the water towards the
mouth. The cilia also act as respiratory organs, by changing the water
with which their bodies are in contact. In some of the species there are
stout bristles or setæ, by which they are enabled to crawl upon
water-plants.
On carefully examining the bodies of the Infusoria, rounded granular
spots will be seen, frequently containing minute Algæ, &c. (fig. 28
_b_). These spots are the digestive cavities, and have been called
_gastric sac´culi_; but whether they are definite sacs or mere
excavations, formed by the particles of food having been forced into the
softer internal substance of the body, has not been positively
determined. The sacculi may be filled artificially by mixing very fine
indigo, or carmine, on a slide with the water in which the Infusoria are
contained. A definite food-tube or alimentary canal has been detected in
a few of the Infusoria; but it cannot be shown to exist in the majority
of them.
A mouth exists in most of them, and is sometimes indicated by a row or
set of cilia somewhat larger than those existing upon other parts of the
body, and leading to or placed near it. The particles of food which have
entered the body are often seen to pass round it, as if circulating,
descending on one side and ascending on the other.
In addition to the gastric sacculi, certain clear transparent spots may
also be seen within the body, appearing light or dark according to the
adjustment of the focus. If these are attentively watched, they will be
seen to contract and finally disappear, becoming again distended and
vanishing at tolerably regular intervals. These are the _contractile
vesicles_ (figs. 27 _a_, 28 _a_, 37 _a_), and they contain a clear
liquid, the nature of which is uncertain.
In many of the Infusoria is a round or elongate granular body (figs. 31
_a_, 36 _a_), which is called the nucleus, the term having been applied
to it from a notion that the Infusoria consisted of simple cells. A
minute red spot is also often seen at the anterior end of the body,
which is supposed to represent an eye, and is called an eye-spot. The
Infusoria are propagated in several ways:--by budding or gemmation, new
beings sprouting out in a bud-like form, usually from the base of the
parent; by division, either transverse (fig. 26 _a_) or longitudinal
(fig. 34 _b_), of the body gradually into two parts, each of which
subsequently becomes a perfect animal; by encysting, the body
contracting into a globular form, and forming a firm coat around it, the
contents becoming resolved into a numerous progeny of young; and by
conjugation and the agency of spermatozoa and ova. We will now proceed
to the examination of a few species, arranging them in the order of the
families to which they belong.
MONAD´INA.--In this family the bodies of the Infusoria are very soft,
and without a skin or integument; they are also exceedingly minute, and
will not admit the particles of indigo.
_Mon´as lens_ (Pl. XI. fig. 16) is very minute, and commonly found in
old infusions. Its body is rounded and flattened, and granular on the
surface. At the front end of the body is a whip-like or flagel´liform
(_flagel´lum_, a whip) filament, differing from a cilium in being rigid
at the base and moveable at the end only, by which it is enabled to row
itself through the water with a wriggling motion.
_Cercom´onas glob´ulus_ (fig. 17) has a spherical body, with two
flagelliform filaments, one arising from the front, the other from the
end of the body. In _Cercomonas crassicau´da_ (fig. 18) the posterior
filament is replaced by a tail-like narrowing of the body.
_Heterom´ita ováta_ (fig. 19) has the body ovate, with two long anterior
flagelliform filaments, one of which is directed forwards, while the
other trails behind.
_Anthophy´sa mül´leri_ (fig. 20) has the monad bodies arranged in little
heads at the ends of an irregularly branched brown stalk. After a time
they become detached and revolve freely in the water.
DINOBRY´INA.--_Dinobry´on sertulária_ (Pl. XI. fig. 21) forms a minute
Sertularia-like polypidom, consisting of rows of cells, each containing
an oval monad with a single anterior filament. The two last species are
common in bog-water.
THECAMONAD´INA.--In these Infusoria the body is inclosed in a firm and
sometimes brittle shell or carapace.
_Trachelom´onas volvoc´ina_ (Pl. XI. fig. 22) has a spherical red shell,
the body being furnished with a single filament and a minute red
eye-spot; while _Chætogléna volvoc´ina_ (fig. 23) has an oblong shell,
covered with little spines.
EUGLÉNIA.--In this family the form of the body is constantly changing,
being at one time spherical, at another fusiform or ovate. It is covered
with a contractile skin or firmer external portion, and has one or more
flagelliform filaments for locomotion. The species are common in
stagnant pools, often colouring the water green or red.
_Eugléna vir´idis_ (Pl. XI. fig. 24) has a spindle-shaped body when
fully expanded, the ends being pale; and at the front end is a red
eye-spot.
_Astásia hæmatódes_ (fig. 25), which is probably a form of the
_Euglena_, is found in stagnant pools, which it renders red. It has no
eye-spot.
ENCHÉLIA.--These Infusoria are found in stagnant water and in
decomposing infusions. The body is covered with cilia variously
arranged, but there is no integument nor mouth.
_En´chelys nodulósa_ (Pl. XI. fig. 26) has a colourless, oblong,
irregularly nodular body, coated with very slender radiating cilia, and
often exhibits numerous vacuoles. It is frequently found undergoing
transverse division (fig. 26 _a_), the body becoming gradually
constricted until it separates into two parts, which become perfect
animals.
_Alys´cum sal´tans_ (fig. 39) has an ovoid-oblong, slightly furrowed
body, surrounded with radiating cilia, and has a side bundle of long
retractile cilia, by means of which it leaps from place to place in the
water.
KERÓNIA.--In this family the body is soft, irregularly ciliated, without
a special integument, but has an oblique row of vibratile cilia leading
to the mouth, and stouter cilia or bristles (setæ) on certain parts of
the body. The sacculi often contain Diatomaceæ, &c.
_Oxyt´richa gib´ba_ (Pl. XI. fig. 27) has a colourless, oblong body,
somewhat expanded in the middle, with setæ at the two ends. In the side
view (fig. 27 _a_), the body is seen to be convex above and flattened
beneath.
PARAMEC´INA.--The species belonging to this family have a soft, flexible
body, which is usually oblong and flattened beneath, with an integument
covered regularly with pits and rows of cilia.
_Col´poda cucul´lus_ (Pl. XI. fig. 30) has a slightly compressed body,
ciliated all over, and kidney-shaped or rounded on one side and notched
on the other, the surface exhibiting rows of nodules. The mouth is
situated at the bottom of the notch.
_Paramécium aurélia_ (fig. 28) has the body oblong or oblong-ovate, the
mouth being placed near the anterior third of its under part. This
infusorium is of comparatively large size, and is often found in
immense numbers in infusions, which it renders milky. It is admirably
adapted for showing the sacculi, which are easily filled with indigo.
The body exhibits two remarkable stellate organs, consisting of a
central contractile vesicle, surrounded by several radiately placed oval
vesicles, which may be seen to contract and dilate with great
regularity. The body is coated with very fine cilia.
_Amphilep´tus fascíola_ (fig. 29) is furnished with an elongate fusiform
or lanceolate flattened body, with a lateral oblique mouth.
_Chil´odon cucul´lulus_ (fig. 37) has an oblong thin body, irregularly
wavy on the sides; the mouth being situated obliquely in front of the
middle, and furnished with a cylinder of parallel rod-like teeth.
_Nas´sula el´egans_ (fig. 31) has the body ovoid or oblong, becoming
globular when contracted, the mouth being furnished with teeth as in
_Chilodon_. It is often found among Oscillatoriæ.
URCEOLARÍNA.--_Vorticel´la convallária_ (Pl. XI. fig. 34) is very
commonly met with in decomposing infusions. The bell-shaped body is
fixed at the end of a slender stalk, which is often seen to be extended
and then suddenly contracted into a spiral (fig. 34 _a_). The cilia are
arranged around a raised rim at the front of the body, and extend down a
fissure leading to the mouth. The sacculi of this infusorium may be
readily filled with indigo. The process of longitudinal division may
also often be observed, taking about an hour for its completion; and
when the new individual is about to separate from the parent, a ring of
cilia may be noticed to have sprung up around the base (fig. 36). The
encysting process is also often visible, the cilia disappearing, and the
body becoming globular and secreting a cyst around it; after a time the
contents become resolved into a number of embryos, which escape by the
bursting of the cyst (fig. 35). In some cases the _Vorticella_ assumes
the form of a _Podoph´rya_ (fig. 40), the surface becoming covered with
tentacle-like processes. This _Podophrya_ was formerly considered a
distinct species.
_Vaginic´ola crystal´lina_ (fig. 33) is contained in a crystalline tube,
from which the body can be protruded. The body is of variable form,
presenting when fully extended a trumpet shape. The cilia exist at the
anterior end, and extend down a lateral fissure as in _Vorticella_. It
is found attached to Confervæ in the water of ponds and bog-pools.
_Sten´tor polymor´phus_ (fig. 38) is a very beautiful trumpet-shaped
infusorium, the body being covered with spiral rows of cilia. The rim is
furnished with stouter cilia, its margins at the notch being spirally
turned inwards. This infusorium is often found in little groups attached
to a gelatinous mass (fig. 38 _c_); and it is met with also in a free or
unattached state.
COLEP´INA.--_Cóleps hir´tus_ (fig. 32) has a barrel-shaped carapace,
transversely and longitudinally furrowed, the furrows being occupied by
cilia. It has two or three spines behind, and ten or twelve at the front
end of the carapace. It is common among Confervæ, and is very voracious,
feeding upon dead Entomostraca, &c.; and if disturbed, at its meal by
moving the cover, it will soon return and resume feeding as before.
_Examination._--The Infusoria must be examined during life; for they are
so altered by preservative liquids that they cannot be well preserved.
The shells of those that are provided with them may be kept simply dried
upon a slide, and in this way a few will retain their form, and the
cilia of all may be more easily distinguished; the vacuoles may also
then be seen very distinctly. When they are confined in a small quantity
of water and are about to die, a curious phenomenon may be observed in
them, a number of oil-like sarcodic globules exuding from the body, and
within these, vacuoles may often be seen to form spontaneously.
The Infusoria may be collected in small phials; but it is difficult to
keep them, as they form the food of the Entomostraca, the Rotatoria, and
the larvæ of insects; so that their enemies are very numerous, and they
soon disappear.
* * * * *
CLASSIFICATION.--Before leaving the subject of living bodies, it may be
well to make a few remarks upon their systematic relation as defined by
classification.
All natural bodies are referable to one of three great kingdoms, viz.
the Animal, the Vegetable, or the Mineral Kingdom. The bodies belonging
to the latter seldom come under the notice of the microscopic observer,
as they are mostly visible to the naked eye, and their minute structure
is the same as that of the larger masses. The general structure of the
members of the vegetable and animal kingdoms has been illustrated in the
preceding pages. These bodies are distinguished from those of the
mineral kingdom by their vital power of appropriating surrounding
matters to their own nutrition and growth--this power being exercised by
their organs, or, in the lowest forms, by any portion of their
substance. Hence animals and vegetables or plants are termed organic
bodies, while minerals are termed inorganic bodies, as having no organs;
and the material of which organic bodies consist is termed organic
matter, that of minerals being inorganic matter. But in both animals and
plants inorganic matter is mixed with the organic matter, having been
taken up or absorbed from the inorganic kingdom, although it does not
usually exist in its characteristic condition, which is that of
crystals, _i. e._ angular solids, as crystals of Epsom salts, &c. The
individual members of the animal and vegetable kingdoms are
systematically divided into certain groups, and these into successively
smaller groups until we arrive at the species. These groups are founded
upon the possession of certain points of resemblance by their members,
forming the distinguishing characters, and their names are significant
and definite. They usually run as follows, the larger and higher groups
standing first in order:--Kingdoms, Subkingdoms, Classes, Orders,
Families, Tribes, Genera, and Species. But it must be observed that the
term species has a different value from that of the other terms; for the
individuals of which the species consist are not only related by
resemblance of structure, but also by their origin--being supposed to
have derived their origin from a parent of original creation; while the
other groups have, as far as we know, no other relation than that of
similarity of structure.
It is obvious that the characters of the various groups might be founded
upon peculiarities of any kind. But on this point two methods must be
specially distinguished, in one of which the groups are founded upon the
sum of all the peculiarities, while in the other they are based upon the
structure of single parts or organs. In the former case the system is
called natural, in the latter artificial. And while the latter often
brings together beings which have perhaps but one or two points of
resemblance, and separates others which are closely related, the former
associates those which are really and naturally similar.
All the groups have special names, so that they may be referred to and
spoken of as in the case of common things; the names being composed of
Greek or Latin words, so that they may be intelligible to all nations;
and as these are dead languages, they will remain good for all time.
In mentioning the name of an animal or plant, the name of the genus is
always used with that of the species; thus, the name of Chickweed is
_Stellaria media_. Because there are mostly several species in a genus;
so that if the name of the genus only were used, the species meant would
be uncertain; and as there are often species of the same name in
different genera, if the name of the species only were used, the genus
meant would be doubtful.
The classification of animals and plants serves two important purposes:
one is that the structural peculiarities and affinities of the groups
may be contrasted and a knowledge of their absolute and their
differential characters acquired, and for this a natural system is
eminently serviceable; the second is that of enabling any animal or
plant to be simply distinguished from any other, for which an artificial
or analytical system is extremely useful.