in suspension millions of the spores (or seeds) of Algæ, ready to adhere and grow as soon as they find a resting place, and these are particularly abundant in the warm season. Whether those of the green kinds, the Chlorosperms, such as the Ulva, Enteromorpha, and various kinds of Confervæ, be more plentiful than others, or whether they are more easily satisfied with a place congenial to their growth, I know not; but these grow most obviously, in the proportion of a thousand to one. Before we have kept our tank stocked a fortnight, its transparent sides begin to be sensibly dimmed, and a green scurf is seen covering them from the bottom to the water's surface, which constantly accumulates, soon concealing the contents of the vessel from distinct observation. On examining this substance with a lens, we find it composed of myriads of tiny plants, mostly consisting of a single row of cells of a light green hue, forming minute threads which increase in length at the extremity; others display small irregularly puckered leaves of deeper green, which develop themselves into Ulvæ, or Enteromorphæ.
If we design the Aquarium to be of any service to us in the observation of its contents, this growth must be got rid of, or we might as well have a vessel with opaque sides. Here then comes in the aid of the Periwink. Exclusively a vegetable-eater, he delights in the green sea-weed, and nothing can be more congenial to his palate than these tender succulent growths. The little Yellow Winkle that I first spoke of, possesses a similar appetite; but he is less suitable for the service required, inasmuch as his constitution
