300 THE ZOOLOGIST. 



for ascent and descent, as a more or less permanent ladder ; it 

 is strengthened by an additional trail of mucus each time a 

 mollusc passes over it, and thus it becomes somewhat strong and 

 lasts for a considerable time. Mr. Tye had young P. hypnorum 

 crawling up and down fixed threads of this kind for eighteen 

 to twenty days together ; and on one occasion he noted three 

 individuals, and a Limnaa glabra, upon a Physd's thread at the 

 same 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 cleverest human gymnast that ever performed. I 

 once saw one ascending, and when it was half-way up the thread it was 

 overtaken by another; then came the 'tug of war'; each tried to shake 

 the other off by repeated blows and jerks of its shell, at the same time 

 creeping over each other's shell and body in the most excited manner. 

 Neither being able to gain the mastery, one began to descend, followed by 

 the other, which overtook it, reaching the bottom first. Yet they are not 

 always bent upon war, but pass and repass each other in an amicable spirit. 

 One of the most beautiful sights in the molluscan economy is to see these 

 little ' golden pippins ' gliding through the water by no visible means ; 

 and when they fight, to see them twist and twirl, performing such quick 

 and curious evolutions, while seemingly floating in mid-water, is astonish- 

 ing, even to the patient student of Nature's wonders." 



This use of threads as more or less permanent ladders is 

 unique, as far as the writer knows, among all the mollusca. 

 Mr. W. Jeffery, who kept Physa hypnorum in an aquarium, has 

 referred to the creature's habit of spinning a thread while rising 

 perpendicularly to the surface ; he notes that after taking in a 

 supply of air it may turn leisurely about and crawl down the 

 same thread ; and mentions that once, while the animal was thus 

 returning, the thread parted from its mooring, "when poor 

 hypnorum was quickly carried to the surface again by the air it 

 had taken in."* Mr. Musson, further, mentions having observed 

 spinning both by P. hypnorum and P. fontinalis f ; and Mr. 

 Standen, who refers to P. fontinalis, has obligingly informed the 

 writer of observations made by him. The last-named naturalist 

 remarks particularly upon the junction of the thread with the 



* Jeffery, ' Journal of Conchology,' iii. (1882), pp. 310-1. 

 f Musson, I. c, 



