SPINNING MOLLUSCS. 295 



LimrKBa, — " In watching the movements of Limncecv 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 pos- 

 sessed 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 Limncea 

 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 con- 

 clusions from observations on Limncea and Planorbis : — "Les Gasteropodes 

 d'eau douce," he says, "pour glisser renverses a la surface de l'eau, com- 

 mencent par prendre appui sur la mince pellicule superficielle qui recouvre 

 toujours l'eau des mares et des etangs; puis ils rampent a la face inferieure 

 cfun mince tapis de mucus que leur pied secrete au fur et d mesure de la 

 progression. Cette locomotion," the author adds, "ne differe de la locomo- 

 tion sur les corps solides qu'en ce sens que, lors de la locomotion aquatique, 

 le Mollusque est reduit a tirer parti de la rigidite de la seule trainee de 

 mucine, tandis que, dans l'autre cas, la trainee est elle-meme collee a 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 Limncea 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 procede employe par les Gas- 

 teropodes d'eau douce pour glisser a la surface du liquide," 'Bulletins de 

 i'Academie Koyale des Sciences, des Lettres et des Beaux-Arts de Belgique' 

 (3), xv. (1888), pp. 421-30; (3) Crowther, "Mucous Tracks of Limncea 

 stagnalis" ' Journal of Conchology,' viii. (1896), p. 230 ; and seejalso Taylor, 

 torn. cit. p. 316, fig. 607. 



