As far back as 1850, Cohn wrote, apparently without any knowledge of what Payen had said before him: "The protoplasm of the botanist, and the contractile substance and sarcode of the zoologist, must be, if not identical, yet in a high degree analogous substances.

Such a structure is of common occurrence among the lowest plants and animals, and is known as a contractile vacuole.

Before the godlike countenance of man appeared on the earth, with its contractile forehead and erectile eyebrows, the answering light of the eye, the expansive nostrils, and subtilely mobile lips; before that the tail was the prime vehicle of emotion and safety-valve of passion.

It has by no means lost its contractile power, for if we apply a strong electric stimulus to it, the fatigue seems to disappear.