298 THE ZOOLOGIST. 



brooks with a rapid current, must find it difficult to visit the 

 surface to breathe, unless, as he suggests, it has the power "of 

 veering out a filamentary cable," by which it can return to its 

 original site.* It is probable, however, that the species of Ancylus 

 are not bound, like the majority of Limnseids, to visit the sur- 

 face; and, in all probability, they do not spin upward threads. 

 For Ancylus lacustris we have a note by Mr. Taylor : — 



11 My valued correspondent, Mr. T. D. A. Cockerell, has communicated 

 to me the interesting circumstance that this species has the power 

 possessed by many other Lymnmdce of spiuning a mucus-thread. He 

 says : 'I have just been watching a young specimen of Ancylus lacustris 

 spinning a downward thread.' According to the rough but characteristic 

 sketch of the circumstance made by Mr. Cockerell, the thread was about 

 half an inch long, attached to the extremity of a leaf of the Anacharis, the 

 body of the animal being bent during the operation, the head and tail nearly 

 close together." -j- 



The sketch referred to was not published by Mr. Taylor ; but 

 the writer is permitted to give a copy of it (Fig. 2). The animal 

 appears to be the only mollusc with a Limpet-like shell known 

 to produce a thread. 



Physid^. 

 The air-breathing freshwater Snails of this family resemble 

 Limnceidce in habits; but they possess greater activity, and make 

 a more general use of threads. Montagu (1803) states that 

 Physa fontinalis i " will sometimes let itself down gradually by a 

 thread affixed to the surface of the water, in the manner of the 

 Limaxfilans from the branch of a tree."§ Here, however, as in 

 some other cases, the animals observed were possibly descending 

 threads already fixed ; for Physids, like Limnreids, are ordinarily 

 slightly lighter than water ; and they spin their threads generally, 

 if not invariably, during ascent. The habit was noticed also in 

 Physa fontinalis by Mr. Warington, who states that on one occasion 

 introducing a rod between the creature and its point of attach- 

 ment, he moved it out of its straight course a considerable 



* Clark, 'Annals and Magazine of Natural History' (2), xv. (1855), 

 p. 285. 



f Taylor, "Ancylus lacustris, a thread-spinner," 'Journal of Concho- 

 logy,' iv. (1883), p. 127. 



| Bulla fontinalis. § Montagu, « Testacea Britannxca,' 1803, p. 227, 



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