52 



vessel, devouring with evident relish the confervoid 

 growth that adheres to the glass. 



The jerking manner in which P. fontinalis and 

 P. hypnorum are in the habit of disporting themselves 

 is doubtless owing to the fact that they spin "mol- 

 luscan threads," like those of the Sphtzriidce, and 

 when suspended in mid-water by these invisible 

 cables, twist themselves about and perform all manner 

 of antics, sometimes in sport and sometimes in angry 

 combat with one another. Nor is the capability of 

 thread-spinning confined to the Sphceriida and 

 Physce ; many other molluscs employ the same 

 means of locomotion. It is well known that Bythinia 

 tentaculata, many, if not all, of the Limnceidce, and 

 several of the slugs are thread-spinners ; but they do 

 not all possess this faculty to the same extent 

 Some species avail themselves of it more frequently 

 than others, and some more frequently in the young 

 than in the adult state. In the ' Quarterly Journal of 

 Conchology' for November 1878, there is a most in- 

 teresting paper on " Molluscan Threads," by Mr. 

 Sherriff Tye, who had read it before the Birmingham 

 Natural History and Microscopical Society. The 

 following is a brief extract from it : 



" Physa hypnorum. As before stated, I have had 

 the young of this species creeping up and down per- 

 manent threads for eighteen or twenty days together. 

 In one case I saw three Physce and a Limncza glabra 

 upon a thread of the former at one time. Often when 

 two Physa meet upon the same thread they fight as 

 only molluscs of this genus can, and the manoeuvres 

 they go through upon their fairy ladders outdo the 



