Limnæus

Limnæus (Gr. limne, 'a swamp'), a genus of pulmonate Gasteropods, living in fresh water in all parts of the world, and feeding on vegetable matter. The shell is thin and pale, capable of containing the whole animal when retracted in danger or buried in the mud during drought. It is a somewhat elongated spire in Limnæus, coiled in one plane in the allied genus Planorbis, and limpet-like in Ancylus. An anatomical peculiarity of interest in the adult animal is the persistence on the head-region of a structure directly derived from the embryonic 'velum' (see GASTERPODS). Limnæus and other fresh-water snails often float and glide shell downwards at the surface with the foot expanded like a boat, the lung-sac being partly used as a hydrostatic apparatus. Semper has made numerous experiments on Limnæus, showing how they vary in relation to their surroundings. Thus, he was able to rear a dwarf brood by keeping them in confined vessels. Most fresh-water snails come to the surface periodically to breathe the air directly, and then return to their grazing-grounds beneath; some are said to utilise the air-bubbles on water-plants; while others have become adapted to deep water, remain at the bottom, and use skin and lung-chamber as substitutes for gills. Moreover, in the young forms the lung-sac at first contains water. The eggs, enveloped in a glairy substance, are laid on stones or aquatic plants, and afford convenient opportunities for the study of development. Limnæus is a useful inmate of a fresh-water aquarium, keeping the water clean and unchoked by algae. The numerous and prolific species perform a similar function in ponds and streams, and furnish food for fishes and birds. They are often infested by parasites, of which many complete their life-history in higher hosts. Thus, Limnæus stagnalis lodges for a while the Liver-fluke (q.v.) of the sheep.