§ 217. Among protophytes those exemplified by _Pleurococcus vulgaris_ are by general consent considered the simplest. As shown in Fig. 1, they are globular cells presenting no obvious differentiation save that between inner and outer parts. Their uniformity of figure co-exists with a mode of life involving the uniform exposure of all their sides to incident forces. For though each individual may have its external parts differently related to environing agencies, yet the new individuals produced by spontaneous fission, whether they part company or whether they form clusters and are made polyhedral by mutual pressure, have no means of maintaining parallel relations of position among their parts. On the contrary, the indefiniteness of the attitudes into which successive generations fall, must prevent the rise of any unlikeness between one portion of the surface and another. Spherical symmetry continues because, on the average of cases, incident forces are equal in all directions. [Illustration: Figs. 1, 2, 3.] Other orders of _Protophyta_ have much more special forms, along with much more special attitudes: their homologous parts maintaining, from generation to generation, unlike relations to incident forces. The _Desmidiaceæ_ and _Diatomaceæ_, of which Figs. 2 and 3 show examples, severally include genera characterized by triple bilateral symmetry. A _Navicula_ is divisible into corresponding halves by a transverse plane and by two longitudinal planes--one cutting its valves at right angles and the other passing between its valves. The like is true of those numerous transversely-constricted forms of _Desmidiaceæ_, exemplified by the second of the individuals represented in Fig. 2. If now we ask how a _Navicula_ is related to its environment, we see that its mode of life exposes it to three different sets of forces: each set being resolvable into two equal and opposite sets. A _Navicula_ moves in the direction of its length, with either end foremost. Hence, on the average, its ends are subject to like actions from the agencies to which its motions subject it. Further, either end while moving exposes its right and left sides to amounts of influence which in the long run must be equal. If, then, the two ends are not only like one another, but have corresponding right and left sides, the symmetrical distribution of parts answers to the symmetrical distribution of forces. Passing to the two edges and the two flat surfaces, we similarly find a clue to their likenesses and differences in their respective relations to the things around them. These locomotive protophytes move through the entangled masses of fragments and fibres produced by decaying organisms and confervoid growths. The interstices in such matted accumulations are nearly all of them much longer in one dimension than in the rest--form crevices rather than regular meshes. Hence, a small organism will have much greater facility of insinuating itself through this _débris_, in which