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.[1] 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:—
"My valued correspondent, Mr. T.D.A. Cockerell, has communicated to me the interesting circumstance that this species has the power possessed by many other Lymnæidæ of spinning a mucus-thread. He says: 'I have just been watching a young specimen of Ancylus lacustris spinning a downward thread.' According to the rough but characteristic sketch of the circumstance made by Mr. Cockerell, the thread was about half an inch long, attached to the extremity of a leaf of the Anacharis, the body of the animal being bent during the operation, the head and tail nearly close together."[2]
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.
Physidæ.
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[3] "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."[4] 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
