It might of course be objected to this interpretation that the beginning of the process, namely, the assumption that chance fragments from the food of the fish falling just on the anemone is very improbable; but I once observed that flat rocks washed over by the sea on the Mediterranean coast (not far from Ajaccio) were so thickly covered with green anemones that at first I took the green growth for some strange sea-grass new to me until I had pulled up a little tuft of the supposed plants and identified them as the soft tentacles of _Anthea cereus_. Anemones must be equally abundant in the tropical seas of Java, and a sinking fragment must often alight on the mouth of one of them. Much attention and keen discussion have in the last few decades been focussed on cases of symbiosis between unicellular Algæ and simple animals. A good example is our green fresh-water polyp, _Hydra viridis_ (Fig. 35, _A_). Its beautiful colour is due to chlorophyll, and it was long a matter of surprise that animals should produce chlorophyll, which is a characteristic and fundamental important substance of assimilating plants, until Geza Entz and M. Braun demonstrated that the green did not belong to the animal at all, but to unicellular green Algæ, so-called Zoochlorellæ, which are embedded in the endoderm cells of the polyps in great numbers (Fig. 35, _zchl_). As these algoid cells assimilate, and thus liberate oxygen, their presence is of advantage to the polyp. That--as was at first believed--they also yield nourishment to the polyp I consider very probable, notwithstanding the apparently opposed results of the experiments of so acute an observer as von Graff, for I have seen a large number of these animals thrive for months, and multiply rapidly by budding in pure water which contained no food of any kind. In favour of this view, too, are some observations, to be cited presently, on unicellular animals, in regard to whose nourishment by the zoochlorellæ living within them there can be no doubt at all. [Illustration: FIG. 35. _Hydra viridis_, the Green Fresh-water Polyp. _A_, the entire animal, greatly enlarged. _M_, the mouth. _t_, tentacles. _sp_, testis. _ov_, ovary, both in the ectoderm. _Ei_, a ripe ovum, already green, in process of being extruded. After Leuckart and Nitsche. _B_, section of the body-wall, about the position of the ovary in _A_. _Eiz_, the ovum lying in the ectoderm (_ect_), in which zoochlorellæ (_zchl_), belonging to the endoderm (_ent_), have already migrated through the supporting middle lamella (_st_). _eik_, nucleus of ovum. After Hamann.]