MORE ABOUT WHEEL ANIMALCULES
Microscopic as the wheel animalcules are they yet have been watched
and examined by their admirers to as great a point of intimacy as that
reached by the devotees of insects or of birds. A remarkable fact
about them is that in about 130 different species (out of the 500
known) it has been found that the males are diminutive creatures, about
one-tenth the size of the females, and devoid of digestive canal; in
fact, little more than minute swimming sacs full of spermatozoa. In
one group, that of the crawling Rotifers, to which the common wheel
animalcule, figured in the last chapter, belongs, no male at all has
ever been discovered. They are all females. They are precisely those
wheel animalcules which are known to microscopists for their power of
surviving (like the little water-bears or tardigrades and some other
minute animalcules) the desiccation, or "drying-up" of the water in
which they were living, swimming, and crawling (see Chapters XV. and
XVI.). And it is quite probable that this power of resistance to the
adverse conditions of changing seasons has, in the crawling Rotifers,
taken the place of the production of eggs fertilized by a male. For,
as in the case of the crustacean water-fleas (and of the terrestrial
plant-lice, or aphides and gall-flies), it is found that the female
Rotifers or wheel animalcules, which hatch from fertilized eggs, are
themselves "parthenogenetic," and lay eggs which develop without
fertilization by males–that is to say, are "impaternate." In the case
of the water-fleas these are called "summer eggs," and after one or
more generations of such fatherless females a proportion of males are
produced which fertilize the females hatched at the same period. The
eggs so fertilized acquire a thick shell and are called "winter eggs."
They remain dormant for some months and resist the injurious influences
of winter cold, or, it may be, of drying up and conversion of the
pond-mud into dust, but hatch out when warmer and wetter conditions
return.
This, however, is just what the adult crawling kind of Rotifer can do
in the full-grown state by drawing up her body into the shape of a ball
and exuding a jelly-like or horny coat. So that she has no need of
"winter eggs," and the whole process of forming them and of males to
impregnate them has "dropped out" of the life-history of this special
kind of resistant Rotifers. The minute insignificant males and the
eventual disappearance of males altogether in some races is a subject
which may well occupy the attention of our human "suffragettes." That
the males are minute creatures, less than the thousandth part of
the size of the females, is a fact also ascertained in the case of
some curious marine worms (called Bonellia and Hamingia). The only
other instance of such degradation of the male sex is in some of the
barnacles (discovered by Darwin), in which the big individuals are
of double sex (hermaphrodite). Adhering to the shells of these are
found minute dot-like "supplemental males." It is to be observed that
these are instances where the inferiority of the male is an obvious
measurable fact. In the mammals, the group of vertebrate animals to
which man belongs, the male possesses measurably greater activity and
size than does the female, and is provided with more powerful natural
weapons, such as teeth and horns. He entirely dominates and controls
the female, or a whole company of females, and in no case is there
equality of the sexes, or any approach to it, still less inferiority of
the male. It is, perhaps, a question whether "by taking thought" this
natural inferiority of the mammalian female can be changed.
The survival of Rotifers, especially of a pink-coloured species
(called Philodina roseola), after long drying or "desiccation," has
been experimentally studied. It is found that if the water in which
some are swimming is placed in a watch-glass and allowed to dry up
rapidly the Rotifers are killed, none reappear when after a few hours
fresh water is poured into the watch-glass. But if a few grains of
sand or particles of moss are present from the first in the water the
final drying up takes place more slowly and the Rotifers find their
way between the sheltering fragments, where the water remains long
enough to give them time to form a little gelatinous case, each for
itself. When thus encased they survive, motionless, for months. The
experiment has often been made, and is not in doubt. According to
trustworthy statements, Philodina can thus survive even for so long as
five years. The processes of life are arrested, but the drying has not
proceeded to the extent which is called chemical drying or dehydration.
The tiny Rotifers are still of soft consistence: the protoplasm is
not chemically destroyed. When one is watched with the microscope as
water is allowed to flow round it after several months of dust-like
aridity, it is seen to emerge from its protective case and at once
to commence swimming and searching for food by means of the currents
directed towards its mouth by its so-called "wheel-apparatus." I may
just say that in the case of the slime-mould called "flowers of tan"
the protoplasm dries to the consistency of hard wax, and I have kept
it for years in that state and then revived it by moisture into full
activity and growth. I used also at one time to keep in my laboratory
a supply of the dried yellow lichen from apple-trees, in which one
could always rely upon finding the animalcules called "Macrobiotus" or
"water-bears" ready to be revived from a desiccated condition, after
three or four years passed in that condition.
Many of the Rotifers carry their eggs when ripe extruded from the
body in two bunches or clusters, as is the habit also of the little
microscopic shrimps known as Cyclops. There is a whole group of
Rotifers which fix themselves by the tail, when full grown, to some
solid support. Each then forms a protective tube or case around
itself, from the mouth of which it puts forth its wheel-apparatus
and into which it can retire for protection. Some of the largest and
most beautiful of the wheel animalcules belong to this group of fixed
or sedentary Rotifers. The crown animalcule (Stephanoceros) is one
of these, having what are discs edged with vibrating hairs in the
common Rotifer–here drawn out into a circlet of tapering lobes like
the points of a coronet (Fig. 37 (_bis_), B). Another is the floscule
(Floscularia), in which the wheel-apparatus has the form of five knobs
arranged on a pentagonal disc around the mouth (A in same figure). Each
knob has a bundle of excessively fine, long, stiff, motionless hairs
spreading out from it ready to entangle food particles which may drift
into contact with them. I used to find the stems of the fresh-water
polyp (Cordylophora) of Victoria Dock a sure source of supply of these
fine little creatures. When seen under the microscope as brightly
illuminated glassy florets on a black ground (by what is called "dark
ground illumination") their strange delicacy and beauty cannot be
surpassed. A rare species of floscule (which I have never seen) has
extra-long and fine filaments, each of which shows a fine streaming
current in its substance, and is, in fact, a naked filament of living
protoplasm like one of the ray-like filaments of the sun-animalcules.
[Illustration: FIG. 37 (_bis_).–Three tube-building wheel animalcules.
_A_, Floscularia campanulata. _B_, Stephanoceros Eichhornii. _C_,
Melicerta ringens.]
The most curious of the tube-building Rotifers are those which form
their tubes of little, equal-sized pellets of solid matter–as it were,
"bricks"–which they first form by compacting fine particles in a
special pit on the head and then build them up and cement them together
in rows to form the tube, adding row after row as the animal itself
increases in length (Fig. 37 (_bis_), C). These are known as Melicerta;
and, though some kinds use any minute particles to make their bricks,
one kind is frequent which uses its own excrement for this purpose.
By feeding the little creatures first with food coloured with carmine
and then with blue-stained material, one can obtain alternate rows of
pink and blue pellets, carefully manufactured and laid in position
to build up the growing length of tube. Melicerta has certainly an
extraordinary and economical way of disposing of that refuse which we
larger creatures carefully remove from our habitations and should be
very unwilling to employ as building material. The individuals of one
rare and interesting kind of the tube-builders, after swimming freely
in the youngest stage, settle down together and form their gelatinous
transparent tubes side by side, to the number of fifty or more, in
such a way as to produce a perfect sphere, a twentieth of an inch or
more in diameter, built up of fused jelly-like tubes radiating from a
common centre. The inhabitant of each tube is quite separate from and
independent of his neighbours, but they all protrude their vibrating
wheel-apparatus simultaneously, and cause the glass-like ball to rotate
and travel through the water. Many years ago I found this beautiful
little thing in a small moss-pool (not more than 3 ft. wide), high up
the sloping-side of the north-west section of Hampstead heath, above
the "Leg of Mutton Pond." The well-meant care of the public guardians
of the heath has now drained this region, and my little moss-pools and
the "bog," in which grew the Drosera, or Sun-dew, and the Bog-bean and
such plants, have gone for ever. But we must console ourselves with the
fact that the same progressive expansion of the great city has given
us electric railways, tubes, and tramways by which we can go farther
afield than Hampstead in a few minutes, and still find moss-pools and
the undisturbed glories of ancient swamps and bog-land.
Many of the Rotifers have a pair of ruby-red eyes, and in some of them
there is a minute crystalline lens overlying the red sensitive spot,
which receives the fibres of the optic nerve coming from the brain–one
on each side. It is almost incredible that so minute a creature–often
only the one-fiftieth of an inch long when full grown–should have a
nervous system and special organs of touch (sensory hairs) as well as
eyes, and on the other hand muscles running from one attachment to
another and called into activity by nerves connected with this same
central brain. The pair of branched tubes, which end internally in
flickering "flame-cells" and open externally far back at the vent, are
kidneys. Similar tubes called "nephridia" or little kidneys are found
in many of the smaller animals; the earthworm has a pair in each ring
of its body.
There is little doubt that the wheel animalcules are related in
pedigree to the primitive ancestors of the marine segmented or annulate
worms, which also gave rise to the ringed leg-bearing jaw-footed
creatures with hard skin, called Crustacea, Arachnids and Insects (the
Arthropods). The wheel-apparatus or cilia-fringed discs of the Rotifer
is seen in the young stages of many marine worms, and also in the
young of marine snails, known as the "veliger"–"velum" or "sail" being
the name given to the wheel-apparatus of the young snails (see the
drawing on p. 181). There are very minute marine annulate or segmented
worms (Dinophilus and others), which come near to the Rotifers in many
features, whilst the ringed or segmented character of the body is
obvious in the common wheel animalcule.
The Rotifers are so small that they are built up of very few "cells"
or nucleated units of protoplasm. Many of them are of smaller size
than some of the big infusorian animalcules, which consist of a single
cell. The Rotifers are probably a dwindled pygmy race descended from
ancestors of ten or a hundred times their linear measurement. It
is an important fact that in the possession of a toothed gizzard,
in the hard body-case or cuirass of some kinds, and in Pedalion's
rapidly-moving legs or paddles, fringed with plumose hairs and moved
by that peculiar variety of muscular tissue which is called "striped
muscular tissue," the wheel animalcules give evidence of relationship
to the Crustacea–that is to say, it appears to be probable that they
were derived from the common ancestor of marine worms and Crustacea
before those two lines of descent had diverged.
Rotifera or wheel animalcules are found all over the world, in the
tropics, the temperate zones, the Arctic and Antarctic, and many
species have a world-wide distribution. They occur in fresh waters
and in the sea, in great lakes, in gutters which dry up, in pools in
the polar regions and on high mountains which are solid ice for the
greater part of the year. A few are parasitic, some living on the legs
of minute Crustacea. One which I discovered in 1868 in the Channel
Islands lives in crowds on the skin of a remarkable sea-worm (Synapta),
which burrows in the sand, exposed at low tide. It holds on (as I found
and figured) by a true sucker, which replaces the forked tail of other
commoner Rotifers. It was named "Discopus" by Zelinka, who searched for
it in consequence of my description, and gave a very detailed account
of it. Others are parasitic inside earthworms, and one is found inside
the globe animalcule Volvox! Another causes the growth of warts or
"galls" in a curious kind of Alga called Vaucheria.