Page:Natural History, Mollusca.djvu/100

 active a state as when the creature was in motion. I would add, also, as adverse to the opinion of the French naturalist, that in the Eolis and other floating species I have distinctly observed the action of the foot muscles; the animal, indeed, literally crawls on the under surface of the stratum of air, just as if it were a plate of glass.

The curious habit which these Water-snails have of rising perpendicularly through the water, and the still more curious power of spinning a thread, by the help of which this feat is accomplished, have been already described in the earlier pages of this work. I shall merely add to these particulars of their history, that they lay in summer large oval masses of clear jelly, which they affix to the stalks and leaves of submerged plants. Each mass contains from 100 to 130 eggs, which are hatched in sixteen days.

This is an immense family. Between sixty and seventy species belonging to it are enumerated as natives of the British isles, and those which inhabit foreign countries are far more numerous. The technical characters may be thus described. The head and tentacles are capable of being wholly withdrawn into the body, in which state they are covered by the infolded skin as with a sheath; the end of the tail tapers to a point, and is destitute of a gland. The lung chamber is generally in front of the body, with the breathing hole at its hinder part.