These animals were discovered by Lewenhoek, who has described and figured many of the species. Muller has greatly extended our acquaintance with this class by his numerous, accurate, and full de scriptions, and elegant coloured delineations of in fusoria. The labours of Bruguiere in the Encyclop. Illethod. form also a valuable contribution. Baker, Needham, Ledermuller, Roesel, Buffon, Schrank, Baron Gleichen, Spallanzani, Lamarck, Bory de St. Vincent, Prevost, and Dumas, have added greatly to our knowledge of the Infusoria by their interesting observations on the economy of the spe cies, or by their general views of their classifica tion. They are called .'gastrica by Latreille, from their having no internal stomach; Protozoa by Meckel, from their being in all probability the first formed of animals; 3licroscopica by Bory St. Vin cent, from their minuteness. This class has been divided by most writers into three distinct groups, which may be considered as orders, though still founded on unimportant characters, viz. Appendi culata, Membranacea, and Incrassata.
This order comprehends the most highly or ganized species belonging to this class, some of which make a near approach to the simplest of the ciliated zoophytes. They present distinct project ing parts, variously situated on the surface of the body, sometimes extending from the anterior part like tentacula or like feet, and sometimes from the posterior extremity like a simple or a bifurcated tail. These animalcules sometimes multiply with
great rapidity in our fresh water stagnant pools, giving a lively green, red, brown, or other colour to the surface. We have observed the Furcularia viridis, Lam. which often forms a lively green coloured film on the surface of our stagnant ponds, to be affected by light precisely in the same man ner as the Hydra among zoophytes. They seek always the illuminated side of the vessel or of the pond in which they are observed. Lamarck com prehends under this order the Genera, 1. Furcoccrca. 3. Kerona.
2. Cercaria. 4. Triehoda.
The animalcules of this order have a simpler form and structure than the preceding. They have generally a depressed or flattened body, with an ovoidal or circular form. They have not those projecting parts like members seen in the former order. The body is sometimes long, slender and flattened like a membrane, as in some of the Kol poda, in others it is nearly circular, as in the Cy ciidium, in others irregularly ovoidal, as in the Pararmecium, and in others angular, as in the Gonium. In the rsursaria, the absorbing surface of the animalcules is increased by the body being concave or hollow like a purse, which probably also affords a safer position for the development of the reproductive gemmules. This order is com posed of the genera, 1. Bursaria. 3. Paramacium. 5. Gonium.
2. Kolpoda. 4. Cyclidium.