INFUSORIA

the highest cla of Protozoa, with many members which are among the most familiar of the group. Their name is due to the fact that, where any infusion of vegetable matter is exposed to the air, Infusoria abound therein in a short time. As the other cla es of Protozoa are distinguished by the presence or absence of characteristic loco~ motor organs, so the infusorians are known by the coating of fine, short, vibratile hairs or cilia; which, present often in hundreds or even you sands, enable them to swim through the water or glide over solid substances. The free swimming forms are generally round or oval in shape, while the gliding forms are. flattened, with a permanent creeping surface; but the e ential structures are the same—a protoplasmic body formed of external ectoplasm and internal endoplasm, containing usually two nuclei (macronucleus and micronucleus), food vacuoles and numerous contractile vacuoles, and furnished frequently with a mouth and cesophagus which guide the food to the endoplasm. As is often the case in: Protozoa, the bodies of many Infusoria are preserved from desiccation in continuous dry weather by the formation of a protective covering—encystment—but this proce is also related to reproduction and to lack of food. When encystment precedes reproduction the contents of the cyst break up into very many minute individuals; but a more usual mode of reproduction is that of simple division, where an individual splits into two—cro wise if free, lengthwise if fixed. Most Infusoria live in the sea or in fresh water, where they swim independently or are attached to rocks, seaweeds, zoophytes, etc., by means of a long contractile stalk or by adhesive organs, or simply by modified cilia. They live on small organisms, such as bacteria, diatoms, and minute Protozoa, which are wafted towards the mouth (where it is present) by the continuous action of the neighboring cilia. Some Infusoria are parasitic, and live in the bodies of other animals, upon the juices of which they feed. The parasitic forms, however, do little apparent harm, with the exception of one form (Ichthyophtheirus), which occasionally causes fatal epidemics amongst fresh-water fishes. The cla Infusoria falls into two distinct groups:— Sub-cla I., Ciliata; in which cilia occur throughout life—some without mouths (A stomata), such as Opalina, parasitic in the food canal of frogs; or the group of Gym no stomata, with simple pore like mouths; or the large permanent which the common slipper animalcule or Paramecium is a member. Or there are forms distinguished by a spiral band of well-developed cilia which leads to the mouth (Spirigera), including the sedentary Stentor, which has a body covered with an even, fine coating of cilia, and which builds a gelatinous tube within which it can withdraw for refuge; the closely related sluggish Balantidium, parasitic in the human food canal; or the stalked forms, such as Fpiséylis, with} a stiff, rigid stalk, or Vorticella, with long stalk which contracts spirally under excitation. Sub-cla IT. Suctoria, with cilia only during the early stages, the functions of these being taken up, in the adults, by attened forms wit ranched arms, as Dendrocomeies, or stalked species with knobbed tentacles, as Acineta.