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pleasing object, especially when viewed by dark back-ground illumination.

A farther step towards mere complex structure is traced in the nearly allied Enteromorpha intestinalis, which, in late summer, forms pale green tubular fronds, often a foot long, in similar habitats, and is found in great abundance in warm pools in the "Black Country." Like Tetraspora, it is a charming object, especially in a young state, and it possesses further interest, as connecting the marine and freshwater floras, being an inhabitant of salt marshes also, and loading up to several species which are exclusively marine.

To the same order as Protococcus belongs that most strange and beautiful of all the lower vegetable organisms, Volvox globator, to which the limits of this paper do not allow more than a passing reference. It must suffice to remark that the analogy between Volvox and Protococcus will be more easily realised by conceiving a number of the green cells of the latter in that motile condition thickly and symmetrically embedded in a larger glassy sphere, with the cilia only protruded.

Volvox should be examined in cells shallow enough to prevent its movement, and by transmitted light, in order to observe its structure; but its most lovely aspect is when it is viewed by powerful dark back-ground illumination in a cell sufficiently deep to allow its exquisite pale green crystal spheres, will their miniature Volvoces within, to maintain their swift and graceful rolling motion across the field of the microscope under low powers, when it becomes an object of matchless beauty.

It is not an abundant plant in this neighbourhood. The ruthless and wanton invasion of Sutton Park by a railway, and consequent filling-up of the well-known "Webb's Stows," have destroyed a constant habitat of this as of several other choice plants, as well as the first known locality for the beautiful Rotifer Melicerta Tyro, discovered there last year by Dr. Hudson.

The distribution of Volvox seems to be capricious, for it often appears in sufficient quantity to render the water pale green, then disappears as suddenly, and perhaps only reappears months or years afterwards,

There are, however, situations in which it appears every year, and where good specimens may be obtained by straining a few pints of the water through muslin, and so, as it were, concentrating the spheres of Volvox, which may then be washed off into a small bottle. If it is desired to keep this plant in cultivation, all Entomostraca must be carefully removed with the pipette, as they devour it greedily.

For details of its structure and mode of reproduction the student should refer to the researches of Busk, Colin, and Williamson, of which a slight summary is given it the "Micrographic Dictionary."

Before passing from the Unicellular Algæ, we must glance at one other large order, which surpasses all others in variety and singularity of form, in perfection of symmetry, and in brilliance of colour—the Desmidiaceæ.