Fig. 3 represents a filament of a _U´lothrix_, which is common in freshwater pools, showing the curious manner in which the endochrome is arranged in the cells, forming bands partially lining the cell-walls. NOSTOCHA´CEÆ.--Two species of the typical genus _Nos´toc_ will serve to represent this family. _Nos´toc commúne_ is found on damp ground or in ponds, and forms to the naked eye firmish, olive-green, skin-like, plaited masses, an inch or more in diameter. Under the microscope it is seen to consist of numerous beaded fibres, imbedded in worm-like gelatinous sheaths; these are curved and interwoven to form the compound mass. In the middle of many of the filaments is an enlarged colourless cell, called the vesicular cell, which is related to the reproduction, but in a manner not yet determined. _Nostoc minutis´simum_ (Pl. VI. fig. 4) forms solid gelatinous bluish-green masses, varying in size from a pin’s head to a pea; it is found upon unhealthy water-plants kept in glass vessels. The component filaments are very slender, wavy, and the sheaths often have a brownish tinge. ULVACEÆ.--These Algæ are mostly marine--some, however, being found in brackish or fresh water, or on damp ground, thatch, moss, &c. They are generally of considerable size, forming flat or tubular fronds, often several inches long, a few being filamentous. They consist of one or more sheets or layers of cells, containing mostly green endochrome. This at first fills the cells, but subsequently becomes converted into single spores, or subdivided into numerous ciliated zoospores. _Ul´va latis´sima_ is very common on the sea-coast, being found attached to stones, shells, &c. It forms a broad, flat, green, rounded or oblong, thin frond, wavy and crumpled at the margins, and from 6 to 18 inches in length. The minute cells form two layers, adherent to each other. The zoospores formed are numerous in each cell. _Enteromor´pha compres´sa_ (Pl. IV. fig. 31) is also common in the sea and in brackish ditches; it is often found floating. The frond is green, tubular, flattened or compressed, and branched, the branches being usually simple and narrowed at the base. The frond consists of two layers of minute cells, separated by a space rendering it hollow. The zoospores are numerous in the cells (fig. 32). PALMELLA´CEÆ.--These Algæ are found in fresh or salt water, or on damp earth, wood, &c. They are green or red, forming round or irregular masses or strata. They consist of loosely connected cells, imbedded in a gelatinous mass or matrix, thus forming a frond. _Chlorococ´cum vulgáre_ (Pl. II. fig. 1) is very common upon the bark of elm-trees, palings, &c., forming a green granular crust. It consists of minute rounded or oval cells, mostly undergoing division into twos, fours, or eights. These cells are attached to the sides or ends of very fine colourless filaments. It is most probable that this organism, which is usually placed among the Algæ, consists of the gonidia of a Lichen.