36 



anchored themselves to copper, and have in consequence 

 imhibed the verdigris. 



PASSING from salt to fresh water, we come to a natural cable 

 which is very common, and yet, on account of its practical 

 invisibility, is almost unknown, except by naturalists. I refer 

 to the curious cable which is constructed by the common 

 Water-snail (Limncea stagnalis}, which has already been 

 mentioned in its capacity of a boat. 



This creature has a way of attaching itself to some fixed 

 object, such as a water-lily leaf, by means of a gelatinous 

 thread, which it can elongate at pleasure, and by means of 

 which it can retain its position in a stream, or in still water can 

 sink itself to the bottom, and ascend to the same spot. This 

 cable seems to be made of the same glairy secretion as that 

 which surrounds the egg-masses which are found so plentifully 

 on leaves and stones in our fresh waters, and, like that substance, 

 is all but invisible in the water, so that an inexperienced 

 eye would not be able to see it, even if it were pointed out. 



Slight, gelatinous, and almost invisible in the water as is 

 this thread, its strength is very much greater than might be 

 supposed. Not only can a mollusc be safely moored in the 

 water by such a cable, but it can be actually suspended in the 

 air, as may be seen from a letter in Hardwicke's Science Gossip 

 for 1875, p. 190 : 



" Last summer (September 29) I met with the following 

 unusual fact. In a green-house, from a vine-leaf which was 

 within a few inches of the glass ... a slug was hanging by 

 a thread, which was more than four feet in length, not unlike 

 a spider-web, but evidently much stronger. 



" The slug was descending by means of this thread, and, as 

 the glutinous matter from the under part of the body was 

 drawn out by the weight of the creature, it was consolidated 

 into a compact thread by the slug twisting itself in the direc- 

 tion of the hands of a clock, the power of twisting being given 

 by the head, and the part of the body nearest the head being 

 turned in the direction of the twist. There was no tendency 

 to turn in the contrary direction. Evidently the thread 

 became hard as soon as it was drawn away from the body. 



" By wetting the sides of slips of glass, I secured two speci- 



