Sphærella lacustris (Girod.) Wittrock. _A_, mature free swimming individual with central red spot. _B_, division of mother individual to form two. _C_, division of a red one to form four. _D_, division into eight. _E_, a typical resting cell, red. _F_, same beginning to divide. _G_, one of four daughter zoospores after swimming around for a time losing its red color and becoming green. (After Hazen.)] =336. Chlamydomonas= is a very interesting genus of motile one-celled green algæ, because the species are closely related to the Flagellates among the lower animals. The plant is oval, with a single chloroplast and surrounded by a gelatinous envelope through which the two cilia or flagella extend. One-celled organisms of this kind are sometimes called _monads_, i.e., a one-celled being. This one has a gelatinous cloak and is, therefore, a _cloaked monad_ (_Chlamydomonas_). The species often are found as a very thin green film on fresh water. C. pulvisculus is shown in fig. 163. When it multiplies the single cell divides into two, as shown in _B_. Sometimes a non-motile palmella stage is formed, as shown in _C_ and _D_. Reproduction takes place by gametes which are of unequal size, the smaller one representing the sperm and the larger one the egg, as in _E_ and _F_. These conjugate as in _G_ and _H_, the protoplasm of the smaller one passing over into the larger one, and a zygospore is thus formed. [Illustration: Fig. 163. _Chlamydomonas pulvisculus_ (Müll.) Ehrb. _A_, an old motile individual; _n_, nucleus; _p_, pyrenoid; _s_, red eye spot; _v_, contractile vacuole; _B_, motile individual has drawn in its cilia and divided into two; _C_, mother plant has drawn in its cilia and divided into four non-motile cells; _D_, pamella stage; _E_, female gamete—egg; _F_, male gamete—sperm; _G_, early stage of conjugation; _H_, zygospore with conjugating tube and empty male cell attached. (After Wille.)]