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The name Protophytes (Protophyta) has been applied to a large number of simple plants, which differ a good deal among themselves. Some of them differ strikingly from the higher plants, and resemble so remarkably certain low forms of animal life as to be quite indistinguishable from them, at least in certain stages. Indeed, there are certain forms that are quite as much animal as vegetable in their attributes, and must be regarded as connecting the two kingdoms. Such forms are the slime moulds (Fig. 5), Euglena (Fig. 9), Volvox (Fig. 10), and others.

Fig. 5.—A, a portion of a slime mould growing on a bit of rotten wood, × 3. B, outline of a part of the same, × 25. C, a small portion showing the densely granular character of the protoplasm, × 150. D, a group of spore cases of a slime mould (Trichia), of about the natural size. E, two spore cases, × 5. The one at the right has begun to open. F, a thread (capillitium) and spores of Trichia, × 50. G, spores. H, end of the thread, × 300. I, zoöspores of Trichia, × 300. i, ciliated form; ii, amœboid forms. n, nucleus. v, contractile vacuole. J, K, sporangia of two common slime moulds. J, Stemonitis, × 2. K, Arcyria, × 4.

Other protophytes, while evidently enough of vegetable nature, are nevertheless very different in some respects from the higher plants.

The protophytes may be divided into three classes: I. The slime moulds (Myxomycetes); II. The Schizophytes; III. The green monads (Volvocineæ).