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: —
"Often, when two Physæ 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 astonishing, 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."[1] Mr. Musson, further, mentions having observed spinning both by P. hypnorum and P. fontinalis[2]; 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
