In many of the Infusoria is a round or elongate granular body (figs. 31 _a_, 36 _a_), which is called the nucleus, the term having been applied to it from a notion that the Infusoria consisted of simple cells. A minute red spot is also often seen at the anterior end of the body, which is supposed to represent an eye, and is called an eye-spot. The Infusoria are propagated in several ways:--by budding or gemmation, new beings sprouting out in a bud-like form, usually from the base of the parent; by division, either transverse (fig. 26 _a_) or longitudinal (fig. 34 _b_), of the body gradually into two parts, each of which subsequently becomes a perfect animal; by encysting, the body contracting into a globular form, and forming a firm coat around it, the contents becoming resolved into a numerous progeny of young; and by conjugation and the agency of spermatozoa and ova. We will now proceed to the examination of a few species, arranging them in the order of the families to which they belong. MONAD´INA.--In this family the bodies of the Infusoria are very soft, and without a skin or integument; they are also exceedingly minute, and will not admit the particles of indigo. _Mon´as lens_ (Pl. XI. fig. 16) is very minute, and commonly found in old infusions. Its body is rounded and flattened, and granular on the surface. At the front end of the body is a whip-like or flagel´liform (_flagel´lum_, a whip) filament, differing from a cilium in being rigid at the base and moveable at the end only, by which it is enabled to row itself through the water with a wriggling motion. _Cercom´onas glob´ulus_ (fig. 17) has a spherical body, with two flagelliform filaments, one arising from the front, the other from the end of the body. In _Cercomonas crassicau´da_ (fig. 18) the posterior filament is replaced by a tail-like narrowing of the body. _Heterom´ita ováta_ (fig. 19) has the body ovate, with two long anterior flagelliform filaments, one of which is directed forwards, while the other trails behind. _Anthophy´sa mül´leri_ (fig. 20) has the monad bodies arranged in little heads at the ends of an irregularly branched brown stalk. After a time they become detached and revolve freely in the water. DINOBRY´INA.--_Dinobry´on sertulária_ (Pl. XI. fig. 21) forms a minute Sertularia-like polypidom, consisting of rows of cells, each containing an oval monad with a single anterior filament. The two last species are common in bog-water. THECAMONAD´INA.--In these Infusoria the body is inclosed in a firm and sometimes brittle shell or carapace. _Trachelom´onas volvoc´ina_ (Pl. XI. fig. 22) has a spherical red shell, the body being furnished with a single filament and a minute red eye-spot; while _Chætogléna volvoc´ina_ (fig. 23) has an oblong shell, covered with little spines.