Owing to the absence of a definite membrane, a distinction between fission and internal cell division can scarcely be made here. Often the cells escape from the gelatinous envelope, and swim actively by means of two cilia at the colorless end (Fig. 12, _B_). In this stage they closely resemble the individuals of a _Volvox_ colony, or other green _Flagellata_, to which there is little doubt that they are related. There are a number of curious forms common in fresh water that are probably related to _Protococcus_, but differ in having the cells united in colonies of definite form. Among the most striking are the different species of _Pediastrum_ (Fig. 11, _D_, _E_), often met with in company with other algæ, and growing readily in aquaria when once established. They are of very elegant shapes, and the number of cells some multiple of four, usually sixteen. The cells form a flat disc, the outer ones being generally provided with a pair of spines. New individuals arise by internal division of the cells, the contents of each forming as many parts as there are cells in the whole colony. The young cells now escape through a cleft in the wall of the mother cell, but are still surrounded by a delicate membrane (Fig. 11, _E_). Within this membrane the young cells arrange themselves in the form of the original colony, and grow together, forming a new colony. A much larger but rarer form is the water net (Fig. 11, _G_), in which the colony has the form of a hollow net, the spaces being surrounded by long cylindrical cells placed end to end. Other common forms belong to the genus _Scenedesmus_ (Fig. 11, _F_), of which there are many species. ORDER II.--_Confervaceæ_. Under this head are included a number of forms of which the simplest ones approach closely, especially in their younger stages, the _Protococcaceæ_. Indeed, some of the so-called _Protococcaceæ_ are known to be only the early stages of these plants. A common member of this order is _Cladophora_, a coarse-branching alga, growing commonly in running water, where it forms tufts, sometimes a metre or more in length. By floating out a little of it in a saucer, it is easy to see that it is made up of branching filaments. The microscope shows (Fig. 13, _A_) that these filaments are rows of cylindrical cells with thick walls showing evident stratification. At intervals branches are given off, which may in turn branch, giving rise to a complicated branching system. These branches begin as little protuberances of the cell wall at the top of the cell. They increase rapidly in length, and becoming slightly contracted at the base, a wall is formed across at this point, shutting it off from the mother cell.