Quarterly Journal of ConcJiology. 405 



require lubricating in their passage through water (as in the case 

 of fishes), but the foot especially, in its passage over the surface of 

 any object. This mucus may readily be seen wlien fresh water is 

 put into any vessel in which mollusks have been kept for a few 

 days, as the bubbles of oxygen then given off by the plants 

 {A/iac/iaris ahinastrum shows it well) adliere to the network of 

 mucus which stretches from leaf to leaf, making it plainly visible : 

 of course the change must be conducted gently. The best plan 

 •is to lift out a bundle of Anacharis from the vessel in which the 

 snails are, and drop it gently into a vessel of fresh water. 



The Slugs possess this mucus-secreting property to a remark- 

 able degree ; each species produces mucus of a color and 

 consistency peculiar to itself, some species being provided with 

 an important slime-gland near the tail. This property is essential 

 ■to their well-being; having no sheltering shell, it serves to keep 

 the body moist and cool in dry weather. Slugs often suspend 

 themselves by a thread, and sometimes use it as a means of ascent. 



The Pectinibranchs, extracting oxygen from the water as it 

 passes over their comb-like gills, are not capable of altering their 

 specific gravity; I have not seen one spin an zipivard thread; but 

 several species, both fluviatile and marine, often ^//.f/tv/^/ themselves 

 from the surface of the water or from a floating object by a thread. 

 The same remarks apply to the Nudibranchs. 



Instances of thread-spinning occur among the Lamellibranchi- 

 ate mollusca. 



The late Dr. Lukis, of Guernsey,* whose name is a household 

 Avord to all lovers of science, and whose kindly heart endeared 

 him to all who ever had the pleasure of his acquaintance, in 

 several most excellent letters to Dr. Jeffreys, which every naturalist 

 may take as models of careful and loving observation, says, in 

 speaking of Sj'^Iicerium lacustre — "When they reach the edge of 



* Jeffreys' " Brit. Con.," vol, i, p. \z et seq. 



