Limnæa.—"In watching the movements of Limnæa in the aquarium," says Mr. Warington, "I was for some time under the impression that they had a power of swimming or sustaining themselves in the water, as they would rise from the bottom of the pond, a portion of the rock-work, or a leaf of the plants, and float for a considerable period, nearly out of their shells," without any apparent attachment. On carefully watching this phenomenon, he found that the creatures "were attached by a thread or web, which was so transparent as to be altogether invisible, and which they could elongate in a similar way to the Spider; they also possessed the power of returning upon this thread by gathering it up as it were, and thus drawing themselves back to the point which they had quitted." The observer mentions a case in which a Limnæa stagnalis, having reached the extremity of a leaf of Vallisneria,
mucus, the authors maintain, just as it does on the attached mucus which it sheds on its path on a solid body. Willem (2), it may be added, evidently unacquainted with the work of Alder and Hancock, has confirmed their conclusions from observations on Limnæa and Planorbis:—"Les Gastéropodes d'eau douce," he says, "pour glisser renversés à la surface de l'eau, commencent par prendre appui sur la mince pellicule superficielle qui recouvre toujours l'eau des mares et des étangs; puis ils rampent à la face inferieure d'un mince tapis de mucus que leur pied'sécrète au fur et à mesure de la progression. Cette locomotion," the author adds, "ne diffère de la locomotion sur les corps solides qu'en ce sens que, lors de la locomotion aquatique, le Mollusque est réduit à tirer parti de la rigidité de la seule trainée de mucine, tandis que, dans l'autre cas, la trainée est elle-même collée à une surface solide." By blowing lycopodium powder on the surface of the water, Willem clearly demonstrated the presence of the floating trail; the grains, gathering into groups on the rest of the surface, adhered evenly to the band of mucus, and showed it distinctly. Under natural conditions this floating trail is usually invisible, but not invariably. We find, for instance, that Mr. Crowther (3), passing along a disused canal connecting bends of the Calder, distinctly saw the tracks of Limnæa stagnalis at the surface of the clear water, in the sunshine, with a darkened background of black mud; they appeared as whitish iridescent paths of mucus, 6–8 ft. long, and half an inch wide, mostly straight, and often crossing one another nearly at right angles. (1) Alder and Hancock, 'Monograph of the British Nudibranchiate Mollusca,' 1845–55, pp. 20–1; (2) Willem, "Note sur le procédé employé par les Gastéropodes d'eau douce pour glisser à la surface du liquide," 'Bulletins de l'Académie Royale des Sciences, des Lettres et des Beaux-Arts de Belgique' (3), xv. (1888), pp. 421–30; (3) Crowther, "Mucous Tracks of Limnæa stagnalis," 'Journal of Conchology,' viii. (1896), p. 230; and see also Taylor, tom. cit. p. 316, fig. 607.
