Page:The Zoologist, 4th series, vol 4 (1900).djvu/328

298 brooks with a rapid current, must find it difficult to visit the surface to breathe, unless, as he suggests, it has the power "of veering out a filamentary cable," by which it can return to its original site. It is probable, however, that the species of Ancylus are not bound, like the majority of Limnæids, to visit the surface; and, in all probability, they do not spin upward threads. For Ancylus lacustris we have a note by Mr. Taylor:—

The sketch referred to was not published by Mr. Taylor; but the writer is permitted to give a copy of it (Fig. 2). The animal appears to be the only mollusc with a Limpet-like shell known to produce a thread.

The air-breathing freshwater Snails of this family resemble Lymnæidæ in habits; but they possess greater activity, and make a more general use of threads. Montagu (1803) states that Physa fontinalis "will sometimes let itself down gradually by a thread affixed to the surface of the water, in the manner of the Limax filans from the branch of a tree." Here, however, as in some other cases, the animals observed were possibly descending threads already fixed; for Physids, like Limnæids, are ordinarily slightly lighter than water; and they spin their threads generally, if not invariably, during ascent. The habit was noticed also in Physa fontinalis by Mr. Warington, who states that on one occasion introducing a rod between the creature and its point of attachment, he moved it out of its straight course a considerable