Page:The Zoologist, 4th series, vol 4 (1900).djvu/329

Rh distance; and, by then slowly drawing the rod upwards, he succeeded in raising the Snail out of the water, a space of about seven inches, suspended by its thread, which, though difficult to see in the water, now became distinctly visible. Mr. Tye chiefly observed Physa hypnorum; he states, however, that P. fontinalis uses its thread in a similar way, though less frequently. According to this observer, the young, as soon as they issue from the egg, are capable of spinning a thread and rising to the surface of the water: —

All the threads observed were spun upwards during ascent to the surface; the longest were the work of P. hypnorum, and were spun in a vessel in which the water was fourteen inches deep; they extended from the bottom to the surface. When a P. hypnorum, spinning its upward thread, was much disturbed, it was seen to abandon the idea of reaching the surface, and to turn and descend the unfinished thread, altering its position, Mr. Tye says, with much dexterity and ease by bringing its extremities together, and changing the point of attachment of the thread from the tail to the head. This is certainly a curious performance; but, allowing for the reversed condition of specific gravity, it will be noted that it exactly corresponds with the turning and ascent of a thread by Limax; the buoyancy of Physa, of course, keeps the thread taut, just as does the weight of Limax. When the animal completes its thread, it attaches it at the surface of the water, a minute concavity at the upper end acting, according to Mr. Tye's description, like a small boat, of air, and sustaining the thread; but it is more correct to say, perhaps, that the thread is continued at the surface in the form of a floating slime-trail, and that, thus anchored, it slightly cups the water's surface-film. When the Physa returns from the surface by descending its thread, Mr. Tye further observes: the thread—not invariably gathered up and carried back—sometimes remains attached to the surface; and in that case, it may be used, both