Page:The Zoologist, 4th series, vol 4 (1900).djvu/320

290 are observable; these are the result of muscular action identical with that giving rise to the creature's ordinary locomotion; and we have here a clue to the nature of the progression through the air and of the thread. Whenever a Slug is in motion it discharges mucus, chiefly from the supra-pedal gland below the mouth; in ordinary locomotion it crawls over a film of this mucus which remains behind as a trail, and when in the air it crawls over a similar film, which collapses into a thread as the animal leaves it; this thread represents the trail in every respect, is derived in the same manner, and is in fact a continuation of it. The thread is lengthened by the continued crawling action, combined with a constant discharge of mucus, and perhaps also by the weight of the animal, which appears to elongate the collapsing film. There is thus no special spinning-organ. The thread, however, is of extreme fineness, and is silky when dry; it generally springs up and floats (remaining attached at the point of origin) when the animal alights; but it sometimes becomes attached to the new support, and is left, marking the animal's aerial course. If the animal does not find a new support, or fall, it sometimes turns upon its thread, ascends it, and regains the former support; it creeps up the thread as on a solid body, the slack (with other mucus emitted during the ascent) accumulating at the tail. It is chiefly when the creatures find themselves exposed to sunshine, dry atmosphere, or other dangers that they crawl from their supports. One doubts, however, whether they derive much advantage from the use of the thread; there is no reason to suppose that they would often be injured if they dropped (as they often do by reason of the imperfection of their spinning). Falls of a few feet do not appear to be harmful; and the writer regards the Slug's spinning as little more than an accidental circumstance resulting from the possession, for ordinary locomotion, of a continuous supply of tenacious mucus.

This faculty of making and using a thread, far from being confined to Land-Slugs, is found to extend not only to shell-bearing Pulmonata, but also to the remaining orders of Mollusca-Gastropoda—Opisthobranchiata, Pectinibranchiata, and Aspidobranchiata (with the possible exception of the last); and the writer's principal object in the present paper is to bring together certain scattered information concerning the so-called spinning