404 Quarterly Journal of Concliology. 



As soon as a young Limnseid issues from the egg it appears 

 to be capable of rising to the surface of the water b}' a thread, its 

 air-sac being no doubt sufficiently charged with air to render it 

 buoyant enough. 



The method of anchoring these threads to the surface of 

 water is singular : a minute concavity at the upper end acts like a 

 small boat, and thus sustains the thread. 



When one of these moUusks descend? by the thread it spun 

 in ascending, it generally carries back the thread with it, gathering 

 it together by a muscular action of the foot, although these threads 

 are sometiines fixed and made to last a considerable time. The 

 longest threads I have seen are those of the Physre, and I have 

 had in a vessel containing fourteen inches depth of water, a 

 number of them fixed by Pliysa hypnorum, up and down which 

 they were creeping for eighteen or twenty days together. I have 

 no doubt they extend their threads to a much greater length, say 

 three or four feet. 



Permanent threads are kept in position and strong enough 

 for use by the addition of a film of mucus each time a mollusk 

 crawls over them; and I may here explain what I wish to convey by 

 saying that the process of spinning is to a certain extent an 

 involuntary act. 



When a snail crav^'ls (either a terrestrial or an aquatic species) 

 it leaves behind it a trail of mucus, which is discharged for the 

 purpose of lubricating the foot in its passage over any surface, and 

 if the continuity of this mucus be not ruptured, we have a thread 

 in all respects analogous to those I am speaking of. 



In the case of an aquatic species, this trail of mucus is 

 usually invisible; hence it may be supposed that mollusks inhabit- 

 ing water do not secrete such a copious supply as their bretheren of 

 the land, and that the water itself would act as a sufficient lubricant; 

 but such is not the case, for not only do the bodies of mollusks 



