or slightly yellow; in some few cases, even with a bluish or greenish tint. The blood-cells are generally colourless, and their number varies according to the richness or poverty of food. In some species, however, as, for instance, in Polyphemus oculus, the blood-cells appeared to be very few in number, or even almost entirely absent.
I subjoin a list of the genera proposed by Prof. Ley dig, with their principal characters:—
SIDA.
Six pairs of legs. The large antennæ with two branches, one of them two-jointed; the other three-jointed.
HOLOPEDIUM.
Characters of Sida. The large antennæ not branched.
LATONA.
Characters of Sida. The large antennæ with three branches.
DAPHNIA.
Five pairs of legs. The large antennæ with two branches; one three-jointed, the other four-jointed.
MACROTHRIX.
Legs and general characters as in Daphnia. The plumose hair of the first segment of the three-jointed branch of the large antennæ is the longest, and its second segment is toothed like a saw.
ACANTHOCERCUS.
Characters generally as in Daphnia. The four-jointed branch of the large antennæ has only three plumose hairs; the three-jointed branch has five, the one belonging to the first segment being the longest.
PASITHEA.
Characters generally as in Daphnia. Both branches of the large antennæ have five setæ. Post-abdomen peculiar; the part lying before the anus small, the posterior portion elongated.
BOSMINA.
Five pairs of legs. Large antennæ two-branched; the one branch with four segments, and three bristles; the other with three segments, and five bristles. The head is produced in front into two long horns.
LYNCEUS.
Five pairs of legs. Stem of the large antennæ short, with two three-jointed branches. Accessory eye large.
