430 Mr. L. Lindinger on 



a passoble length even in climbing up ; in tliecase of Amplii- 

 peplea 1 measured one that was 20 cm. long. 



Apart from the crawling back that occasionally takes place, 

 the same thread is never used more than once ; whether, 

 as might be supposed from Pohl's description, Aple.va hyp- 

 norum behaves differently has yet to be ascertained. The 

 rapidity of its progress is at any rate remarkable. It is true 

 that Aplexa, like its near relation Phjsa, is distinguished by 

 its very active movements. 



Let us now turn to the fact of the thread-spinning itself. 

 This cannot excite surprise when we reflect that all mollusks 

 secrete a viscid mucus, wliich always adheres to the support 

 and is continuously replaced. Anyone who thouglitfully 

 observes a snail crawling away on sandy or dusty ground 

 must notice how clean the animal keeps, although the sticky 

 surface leads us to suppose the opposite (it is not always easy 

 to remove snail slime from tiie fingers). Every collector 

 knows tlie tracks, that cover depressions with a glistening 

 pellicle ; every possessor of an aquarium, who keeps speci- 

 mens of Limncea, must have been vexed at finding that the 

 animals, in so far as they do not eat tliem, regularly glue up 

 some plants (such as MyriophyUum) , so that their siioots 

 look like tlie brush out of the gum -pot when it has become 

 dry. Upon tiiis cleansing process of snails and slugs depends 

 the well-known method of kilUng the animals by means of a 

 repeated distribution of salts or ashes. In consequence of 

 the copious excretion of mucus which is necessary again and 

 again in order to remove the unwelcome and probably also 

 corrosive covering, the animals become so much weakened 

 that they perish *. 



It follows that even the bridge of mucus, alluded to by Baller- 

 stedt, does not occasion surprise. As regards Ballerstedt's 

 assumption that thereby the animals guard themselves from 

 slipping off, before the foot has taken firm hold of the new 

 support, we may entertain two opinions. When the snail 

 passes over a gap in the substratum, the slimy track is bound 

 to form a bridfje. It is self-evident that the latter also safe- 

 guards the animal in a manner that is not to be underrated; 

 its formation is, however, in all likelihood not effected by the 

 animal for this special case, but is the result of the viscid 

 character of the mucus. 



In the progression, too, of mollusks the mucus at all events 



* Further details as to slugs' slime will be found in Kiinkel's paper, 

 "Die Wasseraufnahme bei Nacktschnecken " (Zool. Anzeiger, Bd. xxii. 

 1899, pp. 388-396 and 401-404). 





