The degree of concentration of the water has a marked influence on them. On the 1st October, 1839, after the driest summer on record, the liquid in the tables indicated 25° salinity in Baumé's areometer, and it was of so deep a color as to stain a corner of my pocket-handkerchief a blood-red. On October 28th, after twenty-eight days of steady rain, the water in the pièces maítresses, instead of presenting a purple color, as on the first day of the month, resembled blood with a very large amount of serum, and the monads in it were less numerous, and of a lighter red, although the water was still of 20° salinity.
Finally, we must not omit to state that the monads are very sensitive to light, which they seek with a certain degree of avidity. This may be easily seen by putting a number of these infusoria into a flask two-thirds filled with sea-water. Soon they will be seen to rise to the surface of the liquid, and to crowd together on the side where the light is strongest. If the flask be turned about so as to bring them on the darker side, they soon take their former position again. We must also observe that these animalcules sometimes go down to the bottom of the tables, and then the coloration of the surface grows fainter, or entirely disappears.
From all this it follows that the red color of the Mediterranean salt-marshes is caused by the Monas Dunalii but is that animalcule the only cause of the phenomenon? Has not the Artemia salina of Audouin, Dumas, and Payen, also something to do with it? This problem was soon solved. We have first to bear in mind that these little crustaceans are found in far greater numbers in brackish water than in water at its maximum point of concentration, and that in the latter case, indeed, they occur so rarely that their presence may be regarded as in some sort merely accidental. In water of this kind, the artemia appears to be sickly; it evidently languishes in the over-dense medium; it swims about with difficulty, always keeping at the surface. It is more or less of a red color along the line of its digestive canal; but this coloration is a secondary thing, and is owing to the monads it has swallowed in water. The latter deposits in their intestine salt-crystals, which may be seen through their transparent envelope, mingled with monads in a state of partial or total digestion.
Far, then, from being the cause of the purple tint of salt-water in its last stage of concentration, the artemia is indebted for its accidental coloring to the Monades Dunalii it takes into its digestive canal, or which settle among the filaments of its branchial feet. This I have demonstrated by keeping colorless artemiæ for a while in water tinged by red monads, or simply by carmine, and so giving them a red color.
But, though the artemia has nothing to do with the coloration of water, it is, nevertheless, a subject of wonder and study for the physiologist. Like several other animals belonging to the great sub-kingdom Articulata (psyche, bee, silk-worm moth), our crustaceans possess the singular privilege of reproducing themselves without being sub-