=Mode of Life.= The Algæ (except most of the Bacteria) are themselves able to form their organic material by the splitting up of the carbonic acid contained in the water, or air in some cases, and for this purpose need light. The majority live in water, fresh or salt, but many are present on damp soil, stones, bark of trees, etc. With the exception of the Bacteria, no saprophytes have actually been determined to belong to this group, and only very few true parasites (for instance, _Phyllosiphon arisari_, _Mycoidea_, etc.), but a good many are found epiphytic or endophytic on other Algæ, or water plants, and on animals (for instance, certain _Schizophyceæ_ and _Protococcoideæ_; _Trichophilus welckeri_ in the hairs of _Bradypus_, the Sloth), and several species in symbiotic relation to various Fungi (species of Lichen), to Sponges (_e.g. Trentepohlia spongiophila_, _Struvea delicatula_), and to sundry Infusoria and other lower animals as Radiolarias, _Hydra_, etc. (the so-called _Zoochlorella_ and _Zooxantella_, which are perhaps partly stages in development of various Green and Brown Algæ). =Vegetative Organs.= The cells in all the Algæ (excepting certain reproductive cells) are surrounded by a membrane which (with the exception of the Bacteria) consists of pure or altered cellulose, sometimes forming a gelatinous covering, at other times a harder one, with deposits of chalk or silica formed in it. The cell-nucleus, which in the Schizophyta is less differentiated, may be one or more (_e.g. Hydrodictyon_, _Siphoneæ_) in each cell. Excepting in the majority of the Bacteria, _colour materials_ (of which _chlorophyll_, or modifications of it, always seems to be found) occur, which either permeate the whole cytoplasm surrounding the cell-nucleus, as in most of the coloured Schizophyta, or are contained in certain specially formed small portions of protoplasm (chromatophores). The individual at a certain stage of development consists nearly always of only one cell; by its division multicellular individuals may arise, or, if the daughter-cells separate immediately after the division, as in many of the simplest forms, the individual will, during the whole course of its existence, consist of only a single cell (unicellular Algæ). In multicellular individuals the cells may be more or less firmly connected, and all the cells of the individual may be exactly alike, or a division of labour may take place, so that certain cells undertake certain functions, and are constructed accordingly; this may also occur in parts of the cell in the large unicellular and multinuclear Algæ (Siphoneæ, p. 62).