WHEEL ANIMALCULES
Two hundred years ago the Dutch naturalist Leuwenhoek, who made
many discoveries with the highly magnifying lenses which he himself
ground and mounted, wrote to the Royal Society of London that he had
"discovered several animalcula that protrude two wheels out of the
forepart of their body as they swim, or go on the sides of the glass
jar in which they are living." He says that "the two wheels are thick
set with teeth as the wheel of a watch," and he sent to the society
for publication drawings of these wonderful little creatures. This
was the first account of the Wheel Animalcules. Since then they have
been studied by many microscopists, especially by Ehrenberg, who
figured many in his great book on animalcules in 1838. Fourteen years
later the delightful English naturalist, P. H. Gosse, who studied
and illustrated the "sea-anemones" so ably–and, by his example and
charming descriptions, made the keeping of these beautiful things in
marine aquaria a favourite occupation among people of leisure, blessed
with a "curiosity concerning the things of nature"–published some
microscopical studies on Wheel Animalcules, and continued throughout
his life to make them a special subject of his investigation.
The microscope was greatly improved–in fact, reached its present
state of perfection–during Mr. Gosse's lifetime, and a wonderful
amount was added to our knowledge not only as to the various kinds
of wheel animalcules (which now number not less than 900 species),
but also with regard to the minutest details of their structure,
their growth from the egg, and their habits. Another English lover
of these minute creatures, Dr. C. T. Hudson, of Clifton (Bristol),
began his observations a few years later, and also discovered many
wonderful kinds. It was my good fortune to bring these two devotees of
the Rotifera, or Wheel Animalcules, together, and to induce them to
write a conjoint work on their favourites–after, as they say in their
preface, they had each continued their studies almost daily for thirty
years, and had made innumerable drawings from living specimens, which
are reproduced in the many hundred (mostly coloured) figures engraved
in the thirty-four quarto plates of their monumental book. This was
published in 1889, a year after Mr. Gosse's death at the age of 78. My
friend, Mr. Edmund Gosse, the distinguished man of letters, is the son
of the naturalist; the microscope, the aquarium, and the rock-pools of
the seashore were the familiar delights of his boyhood, as of mine.
In Fig. 34 I have sketched the common Rotifer or wheel animalcule. It
is about the one-fortieth of an inch long. The two specimens drawn
in Figs. 34, A and B, are seen to be clinging by the forked tail-end
of the body to a piece of weed (drawn in dotted lines). The body is
stretched in these specimens to its full length. It can be shortened by
a "telescoping" or pulling in of either end, so as to make the animal
a mere oval particle. The four narrower joints or segments at the
tail-end can be pulled in like the segments of a telescope, whilst the
two wheels and adjacent parts can be drawn down into the body as shown
in Fig. 34, C, where the two wheels (W) are seen showing through the
skin by transparency.
[Illustration: FIG. 34.–Diagram of _Rotifer vulgaris_–the common
wheel animalcule–one hundred and twenty times as long as the creature
itself. _A_, front view. _B_, side view. _C_, head showing eyes _S_,
and retracted wheel apparatus _W_. The letters in _A_ and _B_ have the
following signification: _M_, mouth. _W_, wheel or ciliated disc. _S_,
eye spots on head. _T_, spur or tentacle. _G_, gizzard. _St_, stomach.
_Int_, intestines. _V_, vent: aperture of intestine.]
The common rotifer can walk like a looping caterpillar or a
leech–fixing itself by its tail, then stretching out the head and
fixing that, whilst letting go the tail and bringing it up by
"telescoping" it, near to the head region. The tail is forked, and in
the side view (Fig. 34, B) it is seen to have a soft branched process,
which helps it to cling. The letter V in Fig. 34, A, points to the vent
or opening of the gut at the fork of the tail. The mouth, marked M, is
seen between the two "wheels." The two "wheels" are really two discs,
the edges of which are beset by coarse "cilia," or vibrating hairs of
protoplasm.[5] These cilia "lash" and straighten again one after the
other, so that the optical illusion is produced of the toothed edge of
the disc being in movement like a wheel. They may be "focused" with the
microscope so that the groups or "bunches" of them look like stiff,
motionless "teeth," although they are really, all the time, lashing and
beating in regular rhythm. When the animal is fixed by its tail, the
lashing of the cilia on the wheels causes currents in the water which
set with great strength to the mouth and bring floating food particles
to it. It is thus that the Rotifer feeds. When the tail is not
grasping a support, the movement of the cilia on the wheels causes the
animal to swim forward through the water, so that it has two modes of
locomotion–the leech-like crawling method and the free swimming method.
The various internal organs of a Rotifer are readily seen through its
transparent skin (Fig. 34, A). It has a nervous system, many bands of
contractile muscles and a pair of little tubular kidneys or nephridia,
besides reproductive germs (the eggs). I have here sketched only the
digestive canal. The mouth leads through a gullet to a very curious
organ called the "gizzard," marked G. All the wheel animalcules have
this gizzard, but its teeth, shown as two oval bodies in the drawing,
differ a great deal in shape and complexity in the different kinds.
Whilst the Rotifer is feeding by bringing currents of water to its
mouth, the two halves of the gizzard are kept in rapid movement by
muscles, causing them to rub against one another and to grind up the
food particles which reach them through the gullet. The gizzard (G)
is followed by the digestive stomach (St), and that by the intestine
(Int), which opens at the vent (V). The side (or three-quarter profile)
view of a similar specimen (Fig. 34, B) shows only the surface of
the little animal, and is intended to show especially the snout-like
head-lobe (S), with its two eye-spots, which are red in colour.
Standing out backwards from this is a finger-like process (T), which is
called the spur, or tentacle. It has hairs at its tip, and is a sensory
organ.
[Illustration: FIG. 35.–The Rotifer _Pedalion mirum_–seen from the
right side, magnified 180 diameters. _w.a._, wheel apparatus or
"ciliated" margin of the cephalic disc. _r.e._, right side eye-spot.
_m._, mouth. _p._, tactile process. _d.l._, median dorsal limb (as it
is seen in profile, only three of the fringed hairs at its extremity
are seen). _v.l._, the great ventral limb (only five of its fan of
eight fringed hairs are seen). _l.l._^1, dorso-lateral, and _l.l._^2,
ventro-lateral limbs of the right side: they show the complete fans of
eight fringed hairs. _x._, the pair of posterior processes tipped with
vibratile cilia, better seen in Fig. 36.]
[Illustration: FIG. 36.–The Rotifer _Pedalion mirum_–seen from the
ventral surface. Letters as in Fig. 35. The complete fan of eight
fringed hairs terminating the great ventral limb are seen, and the
three spine-like processes on each side of it. The fringed hairs of the
two ventro-lateral limbs, _l.l._^2, are omitted; they are fully shown
in Fig. 35, and are the same in number and disposition as those forming
the "fan" of the great ventral limb. Compare these hairs with those of
the "Nauplius" Crustacean larva drawn as a tail-piece to Chapter XIII.]
In some wheel animalcules there is a pair of these spurs, and the very
remarkable wheel animalcule drawn in Figs. 35 and 36 has six large
processes which, though much bigger, appear to be of the same nature.
Of these four are seen in Fig. 35, namely, _d.l._, the dorsal limb,
_v.l._, the great ventral limb, and _l.l._^1 and _l.l._^2, the two
lateral limbs of the right side, all of them carrying fanlike groups
of fringed hairs. They are moved by very powerful muscles, and strike
the water with energetic strokes, so as to cause the little owner to
dart through it. This jumping or darting wheel animalcule is called
"Pedalion," and was discovered and described by Dr. Hudson. It is so
astonishing and wonderful a little beast, that when Dr. Hudson sent me
some alive in a tube by post in 1872, soon after he had discovered
it, I could not believe my eyes, and thought I must be dreaming. It
is very like the young form of Crustaceans known as a "Nauplius" (see
tail-piece to the present chapter) in having (what no other wheel
animalcule has) great hollow paired limbs moved by _striated_ muscular
fibre, carrying fringed hairs only known before in Crustaceans (crabs,
shrimps and water fleas), and striking the water violently just as do
those of the Nauplius. And yet all the while it has on its head a pair
of large ciliated wheels which serve it just as do those of the common
Rotifer. No Crustacean, young or old, has this "wheel-apparatus" nor
any vibratile "cilia" on the surface of its body. Pedalion possesses an
astounding "blend" of characters. Fig. 35 shows, besides the "paddles"
or "legs" (of which two on the other side of the animal are not seen),
the broad and large wheel-apparatus W (within which the right eye-spot
_r.e._ is seen), and a little lobe (_p_) called the "chin" lying just
below the mouth (_m_). The big leg (_v.l._) and the pair on each side
(_l.l._^1 and _l.l._^2), of which that on the right side only is seen,
end in beautiful fringed hairs, which are only seen elsewhere in the
Crustacea (water-fleas and others). Those on the lateral limbs and the
great ventral limb (Fig. 36) are set in two groups of four on each side
of the free end of the limb, whilst those on the dorsal leg (_d.l._)
are apparently not so numerous. I have corrected the drawings, Figs.
35 and 36, by reference to actual specimens kindly given to me by Mr.
Rousselet.
[Illustration: FIG. 37.–The Rotifer _Noteus quadricornis_–to show its
curious four-horned carapace–from which the wheel apparatus, _wa_,
emerges in front, and the tail, _t_, behind; somewhat as the head and
tail of a tortoise emerge from its protective "box" or carapace. The
ridges on the horney covering of the Rotifer recall the horney plates
of the tortoises and turtles.]
The 500 different species of Wheel Animalcules or Rotifera differ from
one another in the exact shape of the wheel-apparatus, in the jointing
of the body and its general shape, and in the development, in some,
of a hard skin or shell like a turtle's or tortoise's shell (Fig. 37)
over that broadest region of the body in which in our Fig. 34, A,
the stomach marked "St" is placed. They differ also in the shape of
the gizzard's teeth, in the presence of paddles or legs (in Pedalion
alone), and in the presence in some of longer or shorter projecting
movable rods or bristles in pairs or in bunches. Many build for
themselves tubular habitations of jelly or of hard cemented particles.
They are all minute (from the ¹/₁₂ to the ¹/₅₀₀ in. in length). They
are divided into five principal groups, which are (1) the crawlers,
like the common Rotifer (Fig. 34), which can crawl like a leech and
also swim freely by aid of their wheel-apparatus; (2) the naked free
swimmers, which do not crawl, but move only by swimming; (3) the
turtle-shelled free swimmers (Fig. 37) like the last, but provided with
strong, often faceted, angular, and spike-bearing shells or "bucklers,"
from which head and wheel-apparatus project in front and narrow tail
behind; (4) the rooted or fixed forms (Figs. 37 _bis_); these never
swim when full grown, but each forms and inhabits a protective tube or
case; (5) the skipping or darting forms. Of these there is only the
Pedalion mirum (Figs. 35 and 36), which is quite unlike all the other
wheel animalcules in having limbs like those of the minute water-fleas
(Nauplius, Cyclops) which strike the water and are fringed with
feather-like hairs.
[Illustration: The larval or young form of Crustacea known as "the
Nauplius." This is the "Nauplius" of a kind of Prawn. The three pairs
of branched limbs are well seen. Much magnified.]
FOOTNOTE:
[5] For some account of "cilia," see "Science from an Easy Chair,"
Figs. 29, 33, 40 and the accompanying text.