50 



HARDWICKE'S SCIENCE-GOSSIP. 



oftener tliau any other of the Gasteropoda, 

 especially the aquatic members of the group, and as 

 their method of using them differs from the Pedini- 

 branchiata — water-breathers — we will consider them 

 first. 



In order to be better understood, let us describe 

 briefly their process of respiration. On the side of 

 the creature is situated a sac, or branchial chamber, 

 formed by a fold in the mantle, and having an 

 opening outwards, which the animal can open and 

 shut at will. The air in this sac is renewed by 

 diffusion while the mollusk is at the surface of the 

 water, which air oxygenates the blood through the 

 veins, which ramify in an arborescent form over the 

 roof of the cavity. Now it will be obvious to the 

 reader that when this sac is distended with air, the 

 creature becomes of less specific gravity than water; 

 hence it will float, even against its own will, when 

 dislodged from its hold ; and, on the other hand, 

 when the air in its branchial chamber is exhausted 

 by natural respiration, or expelled by reason of some 

 annoyance, the creature, becoming heavier than 

 water, at once sinks to the bottom ; and on this 

 simple fact hangs the capability of the mollusk to 

 spin an upioanl or downward thread. 



I have never seen a member of this order descend 

 by a thread unless it had first ascended by one, in 

 which case it might return upon the same thread. 

 It would no doubt be possible for it to descend by 

 a thread if its air-chamber was sufficiently empty 

 to allow of its sinking ; but, atmospheric air being 

 essential to the creature's existence, it very rarely 

 voluntarily descends without a supply, and never in 

 such a case by a thread, although it will creep 

 about in the water when the air in its branchial 

 cavity is sufficiently exhausted to allow it to fall 

 to the bottom of the water when loosed from its 

 hold. 



As soon as a young Limnseid issues from the &gg 

 it appears to be capable of rising to the surface of 

 the water by a thread, its air-sac being no doubt 

 sufficiently charged with air to render it buoyant 

 enough. 



The method of anchoring these threads to the 

 surface of water is singular : a minute concavity at 

 the upper end acts like a small boat — of air, and 

 thus sustains the thread. 



When one of these mollusks descends by the 

 thread it spun in ascending, it generally carries 

 back the thread with it, gathering it together by a 

 muscular action of the foot, although these threads 

 are sometimes fixed and made to last a considerable 

 time. The longest threads I have seen are those of 

 the Physse, and I have had in a vessel containing 

 fourteen inches depth of water, a number of them 

 fixed by Physa hypnorum, up and down which they 

 were creeping for eighteen or twenty days together. 

 I have no doubt they can extend their threads to a 

 much greater length, say three or four feet ; but. 



owing to some difficulty in constructing a vessel of 

 such a depth convenient for observation, I have not 

 been able to verify my belief. 



Permanent threads are kept in position and strong 

 enough for use by the addition of a film of mucus 

 each time a mollusk crawls over them ; and I may 

 here explain what I wish to convey by saying that 

 the process of spinning is to a certain extent an 

 involuntary act. 



t When a snail crawls (either a terrestrial or an 

 aquatic species) it leaves behind it a trail of mucus, 

 which is discharged for the purpose of lubricating 

 the foot in its passage over any surface, and if the 

 continuity of this mucus be not ruptured, we have 

 a thread in all respects analogous to those I am 

 speaking of. 



In the case of an aquatic species, this trail of 

 mucus is usually invisible ; hence it may be sup- 

 posed that mollusks inhabiting water do not secrete 

 such a copious supply as their brethren of the land, 

 and that the water itself would act as a sufficient 

 lubricant ; but such is not the case, for not only do 

 the bodies of mollusks require lubricating in their 

 passage through water (as in the case of fishes), but 

 the foot especially, in its passage over the surface 

 of any object. This mucus may readily be seen 

 when fresh water is put into any vessel in which 

 mollusks have been kept for a few days, as the 

 bubbles of oxygen then given off by the plants 

 {Anacharis alsinastrum shows it well) adhere to the 

 network of mucus which stretches from leaf to 

 leaf, making it plainly visible : of course the change 

 must be conducted gently. The best plan is to lift 

 out a bundle of Anacharis from the vessel in which 

 the snails are, and drop it gently into a vessel of 

 fresh water. 



The slugs possess this mucus-secreting pro- 

 perty to a remarkable degree; each species produces 

 mucus of a colour and consistency peculiar to 

 itself, some species being provided with an im- 

 portant slime-gland near the tail. This property 

 is essential to their well-being ; having no shelter- 

 ing shell, it serves to keep their body moist and 

 cool in dry weather. Slugs often suspend them- 

 selves by a thread, but do not use it as a means of 

 ascent. The Pectinibranchs, extracting oxygen 

 from the water as it passes over their comb-like 

 gill, are not capable of altering their specific 

 gravity; hence they cannot spin an upward thread; 

 bat several species, both fluviatile and marine, 

 often suspend themselves from the surface of the 

 water or from a floating object, by a thread, but do 

 not ascend by it again. The same remarks apply 

 to the Nudibranchs, 



Instances of thread-spinning occur among the 

 Lamellibranchiate mollusca. Sphcerium lacustre 

 has been observed by the late Dr. Lukis, of 

 Guernsey, to suspend itself below the surface of 

 the water by a filament half an inch in length, the 



