290 THE ZOOLOGIST. 



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 sup- 

 ports. One doubts, however, whether they derive much advan- 

 tage 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 con- 

 tinuous 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 Aspido- 

 branchiata (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 



