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.

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.