Use of Slime-threads in the Marine Mollusca. 359 



motions of the foot during the process being similar to those 

 observed in the species ah-eady mentioned. 



Rissoa parva (Da Costa). — Several specimens of this 

 very common littoral species were placed under observation 

 on the 29th April last. It proved to be more active than its 

 congener R. striata. Again and again, when by smartly 

 tapping the tube a floating individual was caused to sink 

 below the surface on its suspensory thread, it was seen to 

 regain its floating position by climbing up the thread. On 

 one occasion the anterior part of the foot was observed to be 

 brought into contact with the thread as the animal ascended, 

 and the water surface was seen to be drawn downwards into 

 an inverted cone at the point of suspension. The quickest 

 rate of thread-climbing observed in this species was half an 

 inch in one minute. 



Rissoa cingillus (Montagu). — A single specimen of this 

 species along with six specimens of Limapontia nigra, all 

 gathered in rock-pools at Bullock on the 2nd May last, were 

 placed two days later in a glass tube of the usual dimensions, 

 2 inches by f inch. In a short time all of the animals had 

 crawled up the side of the tube and assumed the floating 

 ])osition foot upwards on the water surface. The tube was 

 laid aside for a short time, and when the observation was 

 resumed the Rissoa was found suspended by its thread at a 

 depth of I inch, and vigorously working its foot and tentacles 

 as if engaged in climbing. Half of the Limapontias were 

 missing, only three of the six placed in the tube being visible 

 on the surface. On holding the tube against the light and 

 bringing a hand-lens to bear on the suspended animal I 

 found tliat the three missing Limapontias had attached them- 

 selves to the shell of the Rissoa, which was striving hard to 

 lift itself and its living burden to the surface. Although one 

 of the Limapontias was fully as large as the Rissoa, hardly 

 three minutes had elapsed before the Rissoa had climbed up 

 three-eighths of an inch. At this juncture one of the smaller 

 Limapontias set out climbing the thread in advance on its own 

 account. It had mounted only a short way, however, when 

 it fell back on the Rissoa's foot, and so hampered its action 

 that the animal soon gave up its laborious efforts to reach the 

 surface and sank slowly with its burden to the bottom of the 

 tube. 



Modiolaria discors (Linne). — This common species, 



