General Characters. The sub-kingdom Protozoa, as the name implies, includes the most lowly organised members of the animal kingdom. From this circumstance it is difficult, if not impossible, to give an exhaustive definition, and the following is, perhaps, as exact as the present state' of our knowledge will allow:
The Protozoa may be defined as animals, generally of minute size, composed of a nearly or altogether structureless jelly-like substance (termed "sarcode"), showing no composition out of definite parts or segments, having no definite body-cavity, presenting no traces of a nervous system, and having either no differentiated alimentary apparatus, or but a very rudimentary one.
The Protozoa are almost exclusively aquatic in their habits, and are mostly very minute, though they sometimes form colonies of considerable size. They are composed of contractile, jelly-like protoplasm, often known by the name of "sarcode," which is semi-fluid in consistence, and is composed of an albuminous base with oil-globules scattered through it. Granules are generally developed in the sarcode, and in many cases there is a definite internal solid particle, termed the "nucleus."
In many of the Protozoa the protoplasm is not surrounded by a definite outer envelope,'thus permanently remaining in the condition of a mere "cytode." In other cases, however (Infusoria), such an outer envelope exists and a central "nucleus" is present, when the organism may be compared with a single "cell" in one of the higher animals. The Sponges, again, if they are to be retained in the Protozoa, are "multicellular" organisms.
In no Protozoon are any traces known of anything like the nervous and vascular arrangements which are found in animals of a higher grade. A nervous system is universally and entirely absent, and the sole circulatory apparatus consists ml certain clear spaces called "contractile vesicles," which are found in some species, and which doubtfully perform the functions of a heart. A distinct alimentary aperture is present in the higher Protozoa, but in many there is none; and in all, the digestive apparatus is of the simplest character. Organs of generation, or at any rate differentiated portions of the body which act as these, are sometimes present; but in many cases true sexual reproduction has not hitherto been shown to exist.
The "sarcode," which forms such a distinctive feature in all the Protozoa, is merely undifferentiated protoplasm, not possessing u permanent distinction or separation of parts," but nevertheless displaying all "the essential properties and characters of vitality," being capable of assimilation and excretion, of irritability and of the power of contraction, so as to produce movements, strictly analogous, in many cases, to the muscular movements of the higher animals. In some, too, the sarcode possesses the power of producing an external case or envelope, usually of carbonate of lime or flint, and often of a very complicated and mathematically regular structure.
The power of active locomotion is enjoyed by a great many of the Protozoa; but in some cases this is very limited, and in other cases the animal is permanently fixed in its adult condition. The apparatus of locomotion in the Protozoa is of a very varied nature. In many cases, especially in the higher forms, movements are effected by means of the little hair-like processes which are known as "cilia," and which have the power of lashing to and fro or vibrating with great rapidity. In other cases the cilia are accompanied or replaced by one or more long whip-like bristles, which act in the same fashion, and are known as "flagella." The most characteristic organs of locomotion amongst the lower Protozoa are known as "pseudopodia," and consist simply of prolongations of the sarcodic substance of the body, which can usually be emitted from the greater portion of the general surface of the body, and are capable of being again retracted, and of fusing completely with the body-substance.
Classification Of The Protozoa. The sub-kingdom Protozoa is divided into three classes - viz., the Gregarinida, the Rhizopoda, and the Infusoria, In the Infusoria only is a mouth present, and hence these are sometimes spoken of as the "Stomatode" Protozoa, whilst the two former classes collec-tively constitute the "Astomata."
The following is a tabular view of the divisions of the Protozoa: - .
Class I. Gregarinida.
Glass II. Rhizopoda.
Order 1. Monera.
„ 2. Amoebea.
„ 3. Foraminifera.
„ 4. Radiolaria.
„ 5. Spongida.
Class III. Infusoria.
Order 1. Suctoria. ,, 2. Ci/iata „ 3. Flagellata