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 himself 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 infusoria. 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 i the lower divisions of the animal scale. But i 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 byapecu-| liar arrangement of cilia upon lobes near the i mouth, which, when in a state of active vibration, give to the lobes the appearance of wheels I in rapid motion.

These so-called wheel ani- maleules are, however, so widely different in their plan of structure, and so much higher in their degree of organization than the polygas-trica, that naturalists have very generally sep- arated them from the infusoria, and have placed them among the eutromostracan crustaceans, We shall, however, treat of both in the present article. I. Polygastric Infusoria. It is unfortunate that the name polygastric, or many-stomached, is taken from a supposed peculiarity of the animal, the existence of which in any of the class has been rendered by later researches more than doubtful, and the absence of which in some families is admitted by Ehrenberg himself. By retaining this name, we commit two verbal inconsistencies: 1, in calling animals many-stomached which have probably no stomach at all; and 2, we form a subdivision of these polygastrica, the characteristic of which is the absence of any digestive tube. Following Ehrenberg, then, we base the first great division of the polygastric infusoria on the presence or absence of an alimentary canal.

Those in which it does not exist he calls anentera; those in which it does, entero-dela. Of the anentera, some have the power of protruding a portion of their homogeneous bodies as a foot-like process; and of these some have a shell, or, in scientific phrase, are loricated, others are non-loricated. The former are called arcellina, the later amaebae. Of the remainder, some are furnished with cilia, others arc not. To the former the term dinobryina is applied when they are loricated, and astasiaea when they are not. The non-ciliated, in like manner, are called peridina when loricated, and cyclidina when naked. The enterodela, or polygastrica having a digestive canal, are divided in the same way into two parallel series, as they have or have not a lorica or shell. First in this parallel series are placed those where the orifice of the digestive tube is single; these are vorticellina and ophrydina. Next come those with two orifices at opposite ends of the body; these are enchelia and colepina. Next are those where the two orifices are irregularly placed, the aspidiscina having no shell, the trachelina and ophryocercina each having a shell; the former having a proboscis but no tail, the latter a tail and mouth anterior.

Lastly, those having two ventral orifices: the euplota, where the shell is present, and the kolpoda and oxytrichina, the former moving by cilia, the latter by other organs, neither having a shell. A diagram will perhaps make this classification more intelligible:

Polygastric Animals. Anentera, Having No Digestive Tube

Entebodei.a, Jut ring a digestive tube.

- As to structure, we have already stated that neither nerves nor vessels have been discovered in infusoria; indeed, in the very lowest class, the amaebœ and arcellina, which are by Dujardin called rhizopoda, and by other writers pseudo-poda, we find life manifesting itself almost without organization. The amœba is a jelly-like mass, without determinate shape, in texture nearly uniform, having no integument; in fact, only differing from a mass of jelly in being slightly more fluid in the centre than at the circumference, and having at some point near its surface a vesicle, perhaps only a vacuole, which pulsates pretty regularly. When this creature is about to move, a current of the more fluid central portion is seen tending toward some one point of the circumference; soon a portion of the mass protrudes, it elongates till perhaps double the length of the animal, the mass of whose body then seems to pass into the protruded and elongated portion, and thus locomotion is effected. The mode of taking food is thus described by Kolliker, who studied it in the actinophrys, a genus closely allied to the amoeba, and like it made up of a mass of jelly, portions of which, scarcely differing from the general mass in structure, are protruded in the form of rays: " The mode in which the actinophrys is nourished is one of the highest and most special interest.

Although the creature has neither mouth nor stomach, yet it takes in solid nutriment, and rejects what is indigestible. This miracle, for so it may almost be called, is thus effected: When in its progress through the water the actinophrys approaches any small plant or animal - a minute crustacean, rotifera, the young of Cyclops, or the lower algœ diatomaceœ for instance - as soon as the mass is touched by one of the rays of the actinophrys, it seems to adhere to it; the ray now slowly shortens itself, and draws its prey to the surface of its own body; the surrounding filaments attach themselves to it, bending their points together, and closing over it till it is enclosed on all sides. Gradually a cup-like cavity is formed in the body of the actinophrys, at the base of the ray, and into this the prey is crowded, till, the cavity still growing deeper, the whole mass comes to be imbedded in the very substance of the animal, which gradually closes around and over it, and thus the mass comes to be contained in a cavity or stomach formed for its reception.