Page:The Zoologist, 4th series, vol 4 (1900).djvu/323

Rh the animal is extended, and the lung-sac filled with air, they differ from truly aquatic and truly terrestrial molluscs in being slightly lighter than the medium in which they live; when detached they generally rise to the surface, and from this position they appear to be unable to drop, except when they withdraw into their shells and expel air from the lung-sac. It thus happens that they usually spin upward instead of downward threads—a circumstance in which they differ, as far as the writer has ascertained, from all other molluscs. The process is probably identical with that seen in Limax, but the thread, instead of preventing the animal's fall, prevents its sudden rise to the surface. The animal, gradually raising the anterior part of its foot from the bottom on which it is travelling, crawls upwards through the water upon its slime, which, left behind in the form of a thread, retains the animal as it slowly ascends to the surface, to which, or to the slime-film now deposited there, the thread is fixed; subsequently, crawling down the thread thus fixed, the creature uses it as a means of descent to its former position. Mr. Warington long ago published notes on this subject, but we are chiefly indebted for our information to Mr. Tye. The latter naturalist kept most of the Limnæids of this country in captivity for the purpose of observing their spinning. Some spun both when young and adult, others when young only; and, while some used their threads frequently, others did so rarely or not at all. The observer concludes, however, that all are more or less expert in this respect, and that "in the pellucid stillness of their own domain, when the eye of man is not present to pry into their daily avocations, this beautiful and delicate method of travelling is often used by them." It is maintained by this author, and by Mr. Taylor, that the creatures can spin downward as well as upward threads; and from the observations of these naturalists it certainly appears that when the air in the lung-sac is sufficiently exhausted, the animal is heavy enough, while yet