SPINNING MOLLUSCS. 293 



the animal is extended, and the lung-sac filled with air, they 

 differ from truly aquatic and truly terrestrial molluscs in being 

 slightly lighter than the medium in which they live ; when de- 

 tached they generally rise to the surface, and from this position 

 they appear to be unable to drop, except when they withdraw into 

 their shells and expel air from the lung-sac. It thus happens 

 that they usually spin upward instead of downward threads — a 

 circumstance in which they differ, as far as the writer has ascer- 

 tained, from all other molluscs. The process is probably 

 identical with that seen in Limax, but the thread, instead of 

 preventing the animal's fall, prevents its sudden rise to the 

 surface. The animal, gradually raising the anterior part of its 

 foot from the bottom on which it is travelling, crawls upwards 

 through the water upon its slime, which, left behind in the form 

 of a thread, retains the animal as it slowly ascends to the surface, 

 to which, or to the slime-film now deposited there, the thread is 

 fixed ; subsequently, crawling down the thread thus fixed, the 

 creature uses it as a means of descent to its former position. 

 Mr. Warington long ago published notes on this subject, but we 

 are chiefly indebted for our information to Mr. Tye.* The 

 latter naturalist kept most of the Limnseids of this country in 

 captivity for the purpose of observing their spinning. Some 

 spun both when young and adult, others when young only ; and, 

 while some used their threads frequently, others did so rarely or 

 not at all. The observer concludes, however, that all are more 

 or less expert in this respect, and that "in the pellucid stillness 

 of their own domain, when the eye of man is not present to pry 

 into their daily avocations, this beautiful and delicate method of 

 travelling is often used by them." It is maintained by this 

 author, and by Mr. Taylor,f that the creatures can spin downward 

 as well as upward threads ; and from the observations of these 

 naturalists it certainly appears that when the air in the lung-sac 

 is sufficiently exhausted, the animal is heavy enough, while yet 



* Warington, 'Annals and Magazine of Natural History' (2), x. (1852), 

 pp. 273-6; (2), xiv. (1854), p. 366; Zool. x. (1852), pp. 3634-5; xiii. (1855), 

 p. 4533 ; Tye, Hardwicke's ' Science-Gossip,' 1874, pp. 49-52 ; ' Quarterly 

 Journal of Conchology,' i. (1878), pp. 401-15. 



f Taylor, « Monograph of the Land and Freshwater Mollusca of the 

 British Isles,' i. (1899), pp. 318-9. 





