Page:The Zoologist, 4th series, vol 4 (1900).djvu/338

308 tenacious thread of mucus." It would have been interesting to have had more particulars of this attachment, for, according to Dr. Gibbons, the South African Cyclostomas fix their shells by a brittle pellicle of dried mucus, proceeding from the edge of the columellar lip, a mode of attachment, as he states, wholly different from that of Chondropoma and Tudora, whose flexible silk-like thread, as just mentioned, passes between the outer lip and the operculum.

Gray has listed Littorina with Pectinibranchs capable of making threads, but the writer does not know on what authority. For repose, out of the water, most of these Periwinkles closely fix their shell by a pellicle of dry mucus, compared by Gibbons to that of the Old-World Cyclostomas, and by Jeffreys and others to the attaching film of Helix. Of Lacuna, however, which belongs to this family, Jeffreys states that the creatures "occasionally secrete slimy threads (like the Limax arborum), by which they suspend themselves from the frond or stalk of a seaweed."

The use of threads is presumably general among Rissoidæ—small, often minute molluscs, which swarm on seaweeds and grasswrack in pools and shallows, as well as on and under stones in some of the deeper waters of the coast. It was in 1833 that Gray made an often quoted observation to the Zoological Society of London, that "the animal of Rissoa parva has the power of emitting a glutinous thread, by which it attaches itself to floating