Use of Sliine- threads in the Marine Mollusca. 3G1 



The foregoing observations are, perhaps, sufficient to sliow 

 tliat the practice of climbing by suspensory threads attached 

 to tlie water surface, or, rather, to tlie mucous film supported 

 by that surface, is quite usual amongst the littoral or shallow- 

 water species of our marine Gastropods. Though the diffi- 

 culties in the way of observing the motions of animals usually 

 minute, and in all cases in violent action on an almost in- 

 visible thread, were too great to enable me to demonstrate the 

 precise method of climbing adopted in any of the cases here 

 recorded, there can be hardly any doubt that it was essentially 

 the same as that described by Taylor in the following passage 

 from his * Monograph of the Latid and Freshwater Mollusca 

 of the British Isles.' Speaking of the well-known climbing 

 habit of the land-slug, Limax arborum, he says (page 318) : — 

 " The same mucous filament can also be made use of if 

 necessary to re-ascend to the point of suspension, this being 

 accomplislied by bringing the extremities of the body together 

 and transferring the point of attachment of the suspensory 

 filament from the tail to the head." 



The rates of vertical travel up the sides of tlie graduated 

 glass tubes of 16 out of the 18 species placed under observa- 

 tion were noted with some particularity, and a brief rSsume 

 of the results may be given here. Taking 1 inch as the 

 standard distance, and giving to each species its quickest 

 observed rate of travel, they may be arranged in order of 

 slowness as follows : — First come Rissoa striata, R. parva, 

 and Modiolaria discors, each crawling its inch in 3 minutes ; 

 Trochus tumidus, Shenea planorbis, and Polycera lessonii 

 come next, each with 2 minutes to the inch ; then Cyprcea 

 eurojjcea, with 1^ minute, Littorina obtusata, 1\ minute, and 

 Rissoa cingillus, \\ minute. Next we have Trochus zizyphinus, 

 Limapontia nigra^ and Actoeonia corrugata, each with 1 

 minute to the inch, closely followed by Nassa incrassata, Avitii 

 50 seconds. The elegant little Trochus helicinus takes only 

 35 seconds, and last and quickest of all come Eolis farrani 

 and E. drummondi, each travelling at what may be accounted 

 a dizzy rate f';r a maiine Gastropod, accomplish an inch in 

 15 seconds and 13 seconds respectively. 



All of these rates are rates of climbing rather than of 

 simple travelling, since they were made on a vertical surface 

 of smooth glass, and no doubt were considerably slower than 

 the rates for the same species would have been on a liorizontal 

 surface. Taylor, in his Monograph already quoted from, has 

 been pleased to calculate the mileage rate of several land and 

 freshwater mollusks. Some of the more active land-3luu:3 



