Some of the Infusoria, however, have a far higher organisation, such as the "Rotiferæ," and especially the "wheel animalcule" (fig. 3), which at one time was supposed, absurdly enough, to possess a living wheel turning round rapidly on its head! This wheel, however, proved to be nothing else than a vortex of water produced by rotatory cilia. This and many other mistakes were owing to the imperfect construction of the microscopes of that date. These and their fellow animalcules possess several organs, as a stomach, &c. The protozoa at one time were supposed to possess a number of stomachs, and were thence called "poly-gastric infusoria," but it has now been demonstrated that these beings merely absorb or entangle portions of organic matter into their substance, and that there are no cysts or stomachs for their reception. Some of the Infusoria are able to withstand enormous changes of temperature, without losing their vitality; cold far below freezing point, and heat equal to boiling water does not destroy them, and they may be dried in the most complete manner which chemists can devise, and kept in this dry state for years, and yet, on the application of moisture, return to life. There is no doubt but every inch of air contains the germs of some of the Infusoria in a dry state or their ova, and make up a portion of that fine halo of dust which always appears in the air when a gleam of sunshine is let into a dark place. The second class of the Radiata, the Entozoa, are those beings which inhabit, as parasites, the intestines and other parts of animals. Their history is very obscure, but there seems to be about twenty varieties of these creatures, and a great number of animals have their peculiar Entozoa. The best known in the human subject are the "Ascaris" or thread-worm, the "Lumbricus Teres" or long-worm, and the "Tænia" or tape-worm; this last is jointed, and grows to several yards in length. [Illustration: FIG. 4.--SERTULARIA OPERCULATA (_a_, Natural size, _b_, magnified).] [Illustration: FIG. 5.--FRAGMENT OF SPONGE, MAGNIFIED.] The third class, the Zoophyta, includes four orders. The first of these, the Hydroida, comprise the Tubularia, Sertularia (fig. 4), and Sponges (fig. 5); these are minute gelatinous creatures, secreting a sort of tube in which they dwell. These tubes are congregated into masses of different forms and colours, and, with other structures totally different, are erroneously known by the general name of "sea-weed." The Zoophyte which dwells in these tubes is furnished with a set of cilia forming a sort of plume. Most of these Hydroida are phosphorescent, such are those which, scattered about on the surface of the sea, contribute to that luminosity so frequently seen. [Illustration: FIG. 6.--GORGONIA GUTTATA (Natural size).]