Quarterly Journal of ConcJwIogy. 413 



cedure has been described by Dr. Geo. Johntson.*— "The creature 

 spins a thread of the viscous fluid that exudes from the foot, to 

 check its downward fall, and enable it to gain the pristine site. 

 But suppose the shock has severed their connection, . . the 

 thread is still made available to recovery. In its fall, accidental 

 or purposed, an air-bubble is emitted, which rises slowly through 

 the water, and as the snail has enveloped it with its slime, this is 

 drawn out as the bubble ascends; and now, having a buoy and 

 ladder whereon to climb to the surface, it waits suspended until 

 that bubble comes into contact with the weeds that everyv/here 

 float around." 



If this be correct, we have a water-breathing mollusk using 

 its thread as a means of ascent after having spun it downwards. 

 This would be analogous to the use of the thread by the Slugs. I 

 have not myself seen a member of this order use its thread against 

 the laws of gravitation. 



"\^^ith regard to the spinning of Neritina fluviatilis. This 

 species is an inhabitant of running streams, and will not live long 

 in confinement. Its structure renders it impossible for it to spin 

 an upivard thread, as the nature of its habitat alike precludes it, 

 and as it could not float in running water, it could not therefore 

 spin a downward thready as obtains with other members of its 

 order. While making these observations, I do not discredit Mr. 

 Warington's statement, because, although the act of floating is not 

 a normal one with the creature, it might have performed it as 

 mollusks sometimes do,t when placed under circumstances which 

 allow of it, albeit in their natural condition they could not possibly 

 do it; and if it floated, there is no reason why it should not have 

 spun a downward thread. 



* Johnston, "Introduction to Conchology," p. 134, 1850. 

 \ For an accotint of this habit in Trochus occidcntalis, a deep-sea species, 

 see Jeffrey's "Brit, Cor.," vol. 3, pp. 335 -6. 



