Page:The American Cyclopædia (1879) Volume I.djvu/546

 514 ANIMALCULES classes, families, and orders to which its heter- ogeneous materials properly belong, and the formation of a class to which the name infuso- ria, first proposed by Muller, is now generally applied. To this class we shall confine our- selves, and shall generally use the term infuso- ria, not that it is absolutely accurate, for though the greater number of these animals are devel- oped in infusions, yet this rule is not without some striking exceptions. If a drop of water in which animal or vegetable matter is decay- ing be placed upon the object-holder of a microscope of adequate magnifying power, say 200 diameters, it will be found to swarm with living beings in active and incessant motion. They vary in size from -^ of an inch, when they are just within the limit of unassisted vis- ion, to a minuteness which it tasks the power of the glass to detect. These are infusoria; they abound in every ditch, pond? lake, or riv- er, are equally numerous in salt as in fresh water, have been found in thermal springs of high temperature, and in the melted snow of the Alps and the Andes; in short, wherever water and decaying vegetable or animal matter exist, these infusorial animals will be found in vast numbers. There is no doubt that they are often drawn up into the atmosphere in watery vapor, and borne to and fro by the winds. Many forms are not deprived of life by complete desiccation, and may therefore be mingled with the dust, and in this condition carried about by the winds, to resume their active vi- tality so soon as they chance to fall into water. The suddenness with which they appear in wa- ter, even distilled water, when exposed to the air, furnished the advocates of spontaneous gen- eration with one of their strongest arguments. Infusorial animalcules have neither vessels nor nerves, and are made up of a uniform tis- sue, called by Dujardin sarcode, and by Huxley protoplasm. This is in some classes of nearly uniform consistence; in others the external layer possesses considerably more density than the internal, while in yet others a distinct pel- licle or skin can be made out. They have no true feet ; a few of the very lowest type have the power of protruding portions of their homo- geneous structure in the form of limbs, which they use both for the prehension of their food and for locomotion. In the higher forms the locomotion is by cilia, or very minute hairs. This motion is probably automatic, as it is con- stant day and night, the animal never sleeping, nor appearing to take rest. Yet it certainly has in some cases many of the characteristics of spontaneity, the animal in his rapid course seeming to avoid obstacles; but the subject of the character of the locomotion of these animals is very obscure. Some of these higher forms have a shell or outer coat, called carapace or lorica ; these are spoken of as loricated. We have already intimated that the systematic classification of the infusoria has been a mat- ter of great difficulty. That of Ehrenberg, to which we shall in the main conform, though i possessing great merit, has also very great defects. He includes among his infusorial an- imals very many large and important families which are now known to belong to the vegeta- ble kingdom. His desmidiece are now very generally, we might almost say universally, admitted to be algas ; and his diatomacecB are now also placed in the vegetable kingdom. The classification of Dujardin, though it has some great advantages over that of Ehrenberg, is deformed by a multitude of new terms, or, what is worse, old terms to which he affixes new significations. The two great obstacles which at present forbid even the hope of success in any attempt at systematic classification of infusoria are: 1, the great difficulty of distinguishing the lower forms of animal from the correspond- ing forms of vegetable life ; 2, that of deciding whether a given form is permanent, or whether we have to do with the larvae of an insect, or some one of those forms which crustaceans, polyps, and other of the lower animals assume m the progress of their alternations of genera- tion. A motion apparently spontaneous was formerly supposed to decide the question in favor of an animal nature; but Vaucher of Geneva (1790) proved that a motion not to be distinguished from the spontaneous movements of animals is common in the spores of the simpler aquatic plants, and is indeed nature's provision for their dispersion. That animals absorb oxygen and give out carbon, while plants give out oxygen and absorb carbon, affords in the opinion of many naturalists the desired test. But although this is a very gen- eral, it is not found to be a uniform law. A third distinctive mark, and probably the most useful, is found in the character of their nutri- tive material plants being nourished by inor- ganic, animals by organic food. There are some exceptions to this rule also, but they are not numerous, nor do they greatly detract from its practical value. Agassiz has satisfied him- self that very many of Ehrenberg's genera are germs of aquatic worms, and he suggests that this is probably the true nature of all the infu- soria. Should this idea prove well founded, the most essential changes will of course be necessary in the arrangement of the infusoria if, indeed, it is not found necessary to break up this class altogether, and distribute the in- dividuals of which it is composed throughout the lower divisions of the animal scale. But meanwhile we shall adopt the classification of Ehrenberg, eliminating from it those families on whose vegetable nature the great mass of naturalists are agreed. Ehrenberg divides the infusoria into polygastrica and rotatoria. The characteristic of the former is the appearance of certain internal cavities, which he supposed to be dilated portions of the alimentary canal, or stomachs ; hence their name polygastric, or many-stomached. The rotifera, the so-called wheel animalcules, are distinguished by a pecu- liar arrangement of cilia upon lobes near the mouth, which, when in a- state of active vibra-