Page:The Zoologist, 4th series, vol 4 (1900).djvu/324

294 extended from its shell, to descend through the water, making a thread as it goes, and to remain suspended in the water upon a thread thus made. This, however, it is believed, but rarely happens. Several of the observations quoted below, it is true, imply descent and suspension; but, as the thread is generally invisible, it is possible that the animals, in some cases, may have been descending, or resting, upon threads already spun and fixed during ascent. The animal's ability to ascend or descend is attributed by Mr. Tye wholly to the condition of the lung-sac; the creatures are lighter than water, he says, when the sac is inflated, and heavier than water when the air of the sac is exhausted or expelled. It must be remarked, however, that it is when the air is exhausted that the creatures ordinarily require to ascend, and when the sac is fully inflated that they have to descend. It seems to the present writer that the changes in the creature's specific gravity are largely contributed to by the contraction or extension of the animal itself into or from its shell; and it is probable that the creature, when sufficiently heavy to sink, is usually too much contracted and withdrawn to form a thread. It is interesting to note that Mr. Tye recognizes the fact that here, as in Limax, the thread represents the mucus-trail of ordinary progression, such a trail, though usually invisible in the case of a Limnæid, being always present in the track of the moving animal. On plants in vessels in which molluscs have been kept for a few days, Mr. Tye adds, a network of mucus stretches from leaf to leaf, and is readily apparent when fresh water is put in, the bubbles given off by the plants then adhering to the mucus-lines.