Many marine animals belonging to different groups, e. g. many Radiolaria, many Anthozoa, especially Sea-anemones, some species of Convoluta among Turbellaria are infested with 'yellow cells.' These bodies have a distinct cell membrane, except in the case of the yellow cells of Acanthometridae (Radiolaria), a nucleus, two colouring matters, one of them chlorophyl, which resemble the colouring matters of certain brown algae, and they are capable of living and multiplying after the death and destruction of their hosts. Under the influence of sun-light they exhale oxygen and form starch both when in and when out of their hosts. It appears probable that they are the swarm spores of various olive-green Sea-weeds (Melanospermeae or Fucaceae), rather than a distinct genus of algae (Zooxanthella of Brandt=Philozoon of Geddes). Marine sponges, it may be added, are infested by various algae, green, blue, red, or brown.

The marine Turbellarian Convoluta Schultzii, and several Turbellarians of the fresh water, e. g. Vortex viridis, the Coelenterates Hydra, Spongilla, and many Protozoa, e. g. Rhaphidiophrys viridis, Stentor polymorphous, are of a bright green colour; and it is a matter of much dispute whether the green chlorophyl bodies (the Zoochlorella of Brandt) do or do not belong to the organisms in question.

It appears to be certain from Geza Entz's researches that many Infusoria may be either green or colourless; e. g. Stentor polymorphus is the green variety of Stentor Mulleri. Coleps viridis, Ophridium viride, Vorticella chlorostigma stand in a similar relation to Coleps hirtus, Ophridium versatile, Vorticella campanula. In some instances the green variety is rare. The chlorophyl bodies have a cellulose membrane, a nucleus, two contractile vacuoles, paramylum (starch) bodies, and green chlorophyl as an envelope, or in plates. They multiply outside the body of their host after its destruction and appear to belong to different genera of unicellular algae (Palmella, Tetraspora, Glaeocystis, &c). Under certain conditions they multiply within their host and are digested by it. Miss Sallitt, however, who has examined some of the same Infusoria as Geza Entz, states that the chlorophyl bodies resemble those of Hydra, and consist of a non-nucleate ball of protoplasm, with a cup-like investment of chlorophyl containing protoplasm. They occur in the endoplasm and increase by division. In Vorticella chlorostigma the chlorophyl is diffused through the protoplasm.

If the Volvocina are rightly included among Protozoa, it must be admitted, without reserve, that there are green-coloured animals which are nourished like plants.

Professor Ray Lankester's researches on Hydra and Spongilla establish clearly the fact that the green bodies of those animals have not the value of cells. They are devoid of nucleus and membrane. The same specimen of Spongilla may be green where fully exposed to light, colourless or flesh-coloured where in shade. But the colourless part turns green on treatment with sulphuric acid exactly as does the colourless plant Neottia, and microscopic examination shows that colourless representatives of the chlorophyl bodies are present. These bodies in Hydra closely resemble the corresponding structures in plants, and appear, like them, to multiply by fission. The ovum, which is the only ectoderm cell in which they are present, is at one time devoid of them. Kleinenberg has traced their origin in it from colourless bodies. Hamann, on the contrary, states that they migrate into it from the endoderm; but he appears to start from the point of view of their algal nature, and to be, like some other observers, ignorant of the fact that the chlorophyl bodies of plants multiply by fission.

It may be added that Hydra viridis, unlike Spongilla, when confined in the dark, does not lose its colour, and that in some varieties green angular bodies have been found similar to the colourless angular bodies of H. fusca.

With reference to Vortex viridis, von Graff states that green-coloured examples were observed by him to give origin to colourless young; that green examples, confined in the dark, became colourless in seven days. He also states that the green Mesostoma viridatum sometimes occurs almost devoid of chlorophyl coloured cells. These facts, according to him, point to the algal nature of the green bodies in these animals, and raise a similar presumption with reference to Convoluta Schultzii.

As to the physiological side of the question, it has been proved that, under the influence of light, oxygen is given off by the yellow cells, as well as by most animals of a green colour, by Hydra among the number. Spongilla and Vortex viridis do not seem to have been investigated in this respect. Starch also is formed in the yellow cells; in the chlorophyl bodies of many Protozoa; in the cells of Spongilla, but not in intimate connection with the green bodies; in the green cells of Convoluta Schultzii, while glycogen in plenty occurs in the colourless mesoderm cells, internal to the layer of green cells in this animal. It has been detected also, but not often, in the protoplasm of the endoderm cells of Hydra viridis. Starch vacuoles, however, it must be added, are found in colourless Spongillae in as great abundance as in those of a green colour.

The theory of symbiosis is that, while the animal supplies carbonic acid, and probably nitrogenous matters to the alga, the latter, under the influence of light, yields a supply of oxygen, of starch, and possibly other substances to the animal. Brandt's experiments appear to prove that those species of Sea-Anemones which contain yellow cells can get a supply of oxygen and of food from the algae, and live at least for a longer time under circumstances in which the same species speedily die if deprived of their yellow cells by confinement for a time, e. g. eight weeks in the dark. Convoluta Schultzii, according to Geddes, seeks the lightest spots, and can live without food if exposed to light for four to five weeks. And it has been noted that many green Infusoria and Radiolaria containing yellow cells appear not to require any food supply ab extra. The fungus in the lichen appears to play the part of an animal, and it is supposed that the mycelium surrounding the rootlets of the oak, etc, acts as an intermediary between a rich soil and the rootlets. The subject of symbiosis, however, is one involving many difficulties, and a student who consults the literature of the subject will find discrepancies in the accounts given by different observers.

The literature is a large one, and the sketch given above, the barest outline.

Die Actinien, Hertwig (O. and R.), J. Z. xiii. 1879 (also separate, as pt. i. of Studien zur Blattertheorie, Jena, 1879). Actiniaria, Hertwig, R., Challenger Reports, vi. 1882. Le Attinie, Andres, Fauna und Flora des Golfes von Neapel, ix. pt. i. 1882. Deep-sea and Pelagic Actiniae, Moseley, Tr. L. S. (2) i. 1879. British Sea Anemones, Gosse, i860.

Cereanthus, Von Heider, SB. Ak. Wien, lxxix. Abth. 1, 1879. Edwardsia, Andres, Mitth. Zool. Stat. Naples, ii. 1881. Various genera, Jourdan, A. Sc. N. (6) x. 1879-80.

Siphonoglyphe. Hickson, Ph. Tr. 174, 1883.

Histology. Von Lendenfeld, Z. A. vi. 1883.

Scissiparity. Andres, Mitth. Zool. Stat. Naples, ii. 1882.

General Physiology. Solger, Biol. Centralbl. ii. 1882-83. Sense of smell. Pollock and Romanes, J. L. S. xvi. 1883. Digestion. Metschnikoff, Z. A. iii. 1880; Krukenberg, Vergleich. Physiol. Vortrage, ii. 1883, p. 53. Chromatology. Kruken-berg, op. cit. iii. 1884, p. 125; MacMunn, P. R. S. xxxviii. 1884-85.

Symbiosis. The term and general viezv, De Bary, Tageblatter der Versam-lung der D. Natf. u. Aerzte, Cassel, 1878, p. 121; O. Hertwig, op. id. Freiburg, i. B. Sept. 21, 1883, also sep. with a plate, 'Die Symbiose'; Fischer, Jena. General account. Brandt, Arch. f. Anat. und Physiol. (Physiol. Abth.) 1882.; Id. Mitth. Zool. Stat. Naples, iv. 1883. Cf. Geddes, Nature, xxv. 1882; Brandt; Moseley; Percival Wright; ibid. Geza Entz, Biol. Centralbl. ii. 1882-83. Klebs, Biol. Centralbl. ii. Green bodies of Protozoa. Geza Entz. Biol. Centralbl. i. 1881-82. Kessler, Arch. f. Anat. u. Physiol. (Physiol. Abth.) 1882. Miss Sallitt, Q. J. M. xxiv. 1884. Of Hydra and Spongilla, Ray Lankester, Q. J. M. xxii. 1882. Of Hydra, Hamann, Z. W. Z. xxxvii. 1882; cf. Ray Lankester, Nature, xxvii. 1882-83, and Von Graff, Z. A. vii. 1884. Convoluta Schultzii, Geddes, P. R. S. xxviii. 1879; Barthelemy, C. R. 99, 1884; cf. on Vortex viridis, Von Graff, Z. A. vii. 1884.

Lichens. Sachs, Text-book of Botany, transl. by Vines, ed. 2, 1882, pp. 318-330. Fungus of roots of trees. Nature, xxxiii. 1885-86, p. 212.