Page:The Zoologist, 4th series, vol 4 (1900).djvu/330

300 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 Limnæa glabra, upon a Physa's thread at the same time: —

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 ; 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