FIG. 2.—VOLVOX GLOBATOR.
Dr. Carpenter says, "It is quite certain that the Desmidiaceæ, like the confervoid plants in general, grow at the expense of the inorganic elements which surround them, instead of depending upon other living beings for their subsistence, and that they decompose carbonic acid and give off oxygen under the influence of sunlight. They have the power of generating from these materials the organic compounds which they require for their own development, and these are such as are formed by other undoubted protophytes, as is proved by the application of the appropriate tests." These "Desmidiaceæ" were long considered to belong to the animalculæ, as were the "Diatomaceæ," "Volvox globator" (fig. 2), &c.
FIG. 3.—WHEEL ANIMALCULE.
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.
FIG. 4.—SERTULARIA OPERCULATA (a, Natural size, b, magnified).
FIG. 5.—FRAGMENT OF SPONGE, MAGNIFIED.