HARDWICKE'S SCIENCE-GOSSIP. 



51 



spinning of which occupied the creature tliree 

 jiours. M. Bouchard-Cbantereux has recorded 

 that the young of S. corneum possesses the same 

 power of spinning a thread. I have myself seen 

 the latter^ anchor itself by a mucous filament. The 

 uses of these threads to the Pulmonobranchs appear 

 to be: — 



1st. They enable the mollusk to reach the 

 surface of the water gently when no other means 

 present themselves. 



2nd. It is a much easier method of locomotion. 



3rd. It is a much quicker mode of travelling ; 

 for if the surface traversed be smooth, as the 

 side of a glass vessel, it will take the mollusk 

 twice the time to creep as to float by a thread, 

 while if the surface be uneven, as the side of a 

 pond or the leaves of a plant, it would be longer 

 still in creeping. 



4tli. As a great part of the lifetime of the Lim- 

 nseidte, especially the Physfe, is spent in floating 

 upon the surface of the water, where they feed upon 

 particles of decaying vegetable matter, this property 

 of thread-spinning seems admirably suited to their 

 requirements. 



It enables the slugs to descend from considerable 

 heights, as from branch to branch of a tree, quicker 

 and easier than by the process of creeping. 



Among the Pectinibranchs, it enables the snail to 

 reach the bottom gently, instead of falling roughly 

 or suddenly. It serves the same purpose among 

 the Nudibranehs.* 



The Sphseridae, through their capability of climb- 

 ing and floating, in which exercises they are fond 

 of indulging, especially when young, are enabled 

 to enjoy a more extended range of habitat and food ; 

 and when, during their excursions, they desire to 

 rest, this mucus-cable (always short, generally 

 hardly to be spoken of as of any length, but simply 

 a mucous attachment) keeps them safely moored, 

 while, with foot and siphons withdrawn, they take 

 a short period of repose. 



Having thus far, I hope, succeeded in indicating 

 the "why and wherefore" of moUuscan threads, 

 I will tabulate the species I have seen spin, and 

 those seen by others, commencing with the species 

 that spins oftenest and best, and relate one or two 

 incidents connected therewith. 



PULMONOBRANCHIATA. 



Physa hypnormi. — As before stated, I have had 

 the young of this species creeping up and down 

 permanent threads for eighteen or twenty days 

 together. In one case, I saw three Physse and a 

 Limncea glabra upon a thread of the former at one 

 time. Often, when two Physse meet upon the same 

 thread, they fight as only moUusks of this genus 



* Alder and Hancock, " Monograph of the Nudibranchiate 

 Mollusca." 



can, and the manoeuvres they go through upon 

 their fairy ladders outdo the cleverest human gym- 

 nast that ever performed. I once saw one ascend- 

 ing, and when it was halfway up the thread it was 

 overtaken by another; then came the "tug of 

 war " ; each tried to shake the other off, by repeated 

 blows and jerks of its shell, at the same time creep- 

 ing over each other's shell and body in the most 

 excited manner. Neither being able to gain the 

 mastery, one began to descend, followed by the 

 other, whicli overtook it, reaching the bottom 

 first. Yet they are not always bent upon war, but 

 pass and repass each other in an amicable spirit. 

 One of the most beautiful sights iu moUuscan 

 economy is to see these little " golden pippins " 

 gliding through the water by no visible means ; and 

 when they fight, to see them twist and twirl, per- 

 forming such quick and curious evolutions, while 

 seemingly floating iu mid-water, is astonishing, 

 even to the patient student of Nature's wonders. 



Physa fontmalis stands next as a thread-spinner, 

 usingthe thread in a similar manner, but not so often. 



Limnaa glabra, although not using this means of 

 locomotion so often, nevertheless spins well and 

 easily. 



L. stagnalis is active when young, but its habit of 

 spinning decreases as it grows older. 



L. palustris. — The same remarks apply to this 

 species also, although I have not seen it spin so 

 commonly as stagnalis. 



L.jjeregra. —This species has been observed to 

 spin by my friend Mr. R. M. Lloyd, but it very 

 seldom uses a thread. 



L. glutinosa, recorded as a thread-spinner by Mr. 

 Warington. 



Planorbis complanatits, P. spirorbis, P. con- 

 tortus. — These species spin very much less often 

 than the foregoing. 



Limax arborum. — M. Bouchard-Chantereux has 

 seen young individuals of this species descend from 

 branch to branch of a tree by a mucous filament, 

 and he supposes this species to be the Limax filans, 

 or spinning slug of some English authors of the 

 last century. Mr. Daniel has also seen this species 

 suspended in couples from the branches of trees 

 during the breeding season.* 



L. agresfis uses a thread in a similar manner, f 



L. maxinms has been observed to lower itself a 

 distance of three or four feet by a thread. % 



Megalomastoma suspensum, inhabiting the IV^est 

 Indies, derives its name from its habit of suspend 

 ing itself from the branches of trees by a thread. § 



* Jeffreys, " Brit. Con.," vol. i. pp. 136-/. 



t Ibid., p. 135. 



t Lovell Reeve, " British Land and Fresh- water MoUusks," 

 p. 26. 



§ Guilding, quoted by Woodward, "Manual of the Mol- 

 lusca," p. 209. 



D 3 



