Use of SUme-threads in the Marine Mollusca, 355 



that " most spinning Pectinibranchs no doubt are able to 

 ascend to their former positions by crawling up the suspensory 

 thread : this has been observed in Litiopa, in Valvata, and 

 perhaps in Rissoa.'''' The reference to Rissoa in this con- 

 nexion is apparently drawn from Gray's account of the 

 behaviour of Eissoa parva given in 1833 to the Zoological 

 Society of London, where he states that the animal " has the 

 power of emitting a glutinous thread by which it attaches 

 itself to floating sea-weeds and is enabled when displaced to 

 recover its previous position." Gray, too, in a later commu- 

 nication, published in these ' Annals ' fifty years ago, (3) iv. 

 1859, p. 239, appears to attribute the power of re-ascension 

 to the Opisthobranch Elysia viridis; but in this case, as in 

 his note on Rissoa parva, his language is not quite clear. 



Of the 18 County Dublin species placed under observation 

 by me last year no less than 10 were seen to climb up along 

 their suspensory slime-threads to the water surface from 

 which they had descended ; and I have little doubt that had 

 material and opportunity been forthcoming for further obser- 

 vation, many others of the 18 would have shown themselves 

 to possess the same power. As for the method of observation 

 adopted, in all cases the living animals were placed in 

 graduated tubes or phials of convenient size filled witii fresh 

 sea-water, which was renewed from time to time. For the 

 smaller species tubes 2 inches high by f inch in diameter 

 were used, for the larger species phials 3 inches high by 

 1^ inch. in diameter, so that all of the individuals dealt with 

 had ample water surface to float and travel upon. Several of 

 the species were observed to drop, to all appearance volun- 

 tarily, from the water surface, and hang suspended beneath 

 it by slime-threads; but in order to sliorten the period of 

 observation most of them were induced to assume this position 

 by smartly tapping the bottom of the tube against the table 

 on which it stood. As a rule there was little difficulty in so 

 gauging the force of the tap and of the resultant jar as to 

 dislodge the animal's foot from the slime-raft on wliicii it had 

 travelled out from the wall of the glass tube without altogetiier 

 severing its connexion with that raft and causing the animal 

 to sink to the bottom. 



In all cases the animal was found to hang suspended from 

 the posterior end of the foot, and the slime-thread by which it 

 hung, fine and diaphanous though it was, could usually be 

 detected by holding up the tube and examining the water 

 with a hand-lens, while varying the strength of the light and 

 the direction of its incidence on the tube. The graduation of 

 the tubes and fluids was effected by narrow strips of white 



