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.
CHAPTER XV
SUSPENDED ANIMATION
OUR leading newspapers, with rare exceptions, never report the discoveries announced at our scientific societies. But they often seek to astonish their readers with silly stories of monsters said to have been seen in tropical forests, ghostly "manifestations" and such rubbish transmitted to them at a high price by crafty "newsmongers," and do much harm to themselves and to the public thereby. On the other hand, foreign newspapers do occasionally report the proceedings of their local Academies—and then "our own correspondent" telegraphs to London with a flourish, a confused report of what he has read and ignorantly imagines to be "a startling discovery" because he knows nothing whatever of the subject. Thus shortly before the recent war—the confirmation by a French experimenter of the fact, long since demonstrated, that the seeds of plants can survive exposure to very low temperature, was announced with ridiculous emphasis by one of these "fat boys" of journalism pour épater le bourgeois.
A temperature very near to that of the total absence of that molecular movement or vibration which we call "heat," can now be attained by the use of liquid hydrogen, which enables us, by its evaporation, to come within a few degrees (actually three!) of that condition known as the "absolute zero." We divide into one hundred equal steps or degrees the column of liquid (mercury, spirit, or other liquid) of a thermometer as it expands from the shrunken bulk which it occupies when placed in freezing water to the full length which it attains when the water is heated to boiling point. This is called the centigrade scale, or scale of a hundred degrees. But, as we know by the records of travellers in the Arctic regions and by the experiments made in laboratories, there are "degrees" of coldness or diminution of heat which are much below that of freezing water, and can be measured by the further shrinking of the column of liquid in the thermometer, so that we record "degrees below zero centigrade," each of the same length as those above it and corresponding to the same "quantum" of decrease or increment of heat. As we pass from the temperature at which water is solid to that much lower or diminished state of hotness at which mercury becomes solid, the shrinking column of the thermometer (in which a liquid is used not rendered solid by this amount of cooling) falls through 39 degrees of the centigrade size, so that we say that mercury freezes at minus 39 or at 39 degrees below zero of the centigrade scale. The conclusion has now been reached that the absolute zero or cessation of all heat in a body is represented by a fall of no less than 273 degrees below zero on the centigrade scale. Hydrogen gas becomes a liquid at 252 degrees below zero centigrade, and a solid at 264 degrees. If we start our counting of those degrees or increments of heat, of which there are 100 between the freezing and boiling points of water, at the absolute zero or condition of total absence of heat, we must say that hydrogen "melts"—that is, passes from the solid to the liquid state—at 11 degrees (absolute), and boils at about 20 degrees (absolute), whilst water does not melt until 273 degrees (absolute) of temperature are reached, and boils at 373 degrees above the absolute zero.