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MOLLUSCAN THREAD S.

By G. SHERRIFF TYE.

[ONTAGU, at the beginning of this century, noticed the habit in Physa fontinalis of thread-spinning. He says: "Physa fontinalis spins a filament by which it lets itself down from the surface after floating." Later, Mr. Robert Warington* gave an exceedingly interesting account of this thread-spinning by Limnæa glutinosa, L. stagnalis, various species of Planorbis (not named by him), and Physa fontinalis. The latter upon one occasion formed a thread so tough that he was enabled to lift the snail seven inches above

the surface of the water by it. The author includes in his list of threadspinners Neritina fluviatilis—of this I shall speak further on-and concludes by stating his belief that "all the fresh-water snails are possessed of this power."

Now, after this well-proven fact of spinning, stated upon the authority of so good an observer, you would scarcely expect to find such an observation as this:-"The Physæ, especially P. hypnorum, are active in habit, whether swimming foot uppermost, on the surface of the water, holding themselves stationary at different depths in the water, or gliding through it in sudden jerks by an hydraulic action of the foot. By bringing the lateral margins of this organ into contact, the animal constructs a tube for inhaling and suddenly expelling the water either upwards or downwards.' Montagu stated, and the statement has been repeated by Jeffreys, that the animal spins a mucous thread for letting itself down in the water and rising again for respiration; but I have not succeeded in confirming this observation, and have great doubts of its accuracy."†

Zoologist, 1852, pp. 3634-5; 1855, p. 4533.

+ Lovell Reeve, "British Land and Fresh-water Mollusks," pp. 150-1. 1853.

No. 111.

Mr. Reeve does not tell us how he proved his assertion about the "hydraulic action of the foot," and does not seem to have tried to ascertain how they "hold themselves stationary at different depths in the water,"-coolly "doubts" Montagu's statement about the "mucous thread," and does not notice Mr. Warington's observations at all. I may state that a mollusk is only capable of "holding itself stationary at different depths in the water" when attached to a thread, and that no "hydraulic action" of the foot takes place. When a mollusk is forming a thread, the "lateral margins" of the foot are brought together, forming a channel for the natural flow of mucus down the sides of the foot to the tail; thus adding to the thread, which is gradually extended. The existence of a thread may be proved, as stated by Mr. Warington, by passing a rod under the creature, by which means it can be swayed to and fro.

I have taken great interest in this thread-spin. ning, and long before I had read Mr. Warington's excellent notes I had been observing this seeming phenomenon, and had tabulated the species absolutely seen by myself in the act, and noted the conditions under which mollusks are capable of producing and using a thread.

Let me here explain that the words thread and spinning are used descriptively, and it must not be supposed that these threads, or the production of them, bear any analogy to the spinning of spiders. In the case of the mollusk the thread is gelatinousin fact, is formed of the slime of the creature, the process of forming it being, to a certain extent, an involuntary act, although it is used for a set purpose; whereas the spider's thread is silken, and its formation is entirely under the control of the creature. Neither are they to be confounded with the byssal filaments of the Mytilidæ, Pectinidæ, Dreissena polymorpha, &c., these latter being of a fibrous nature, and the product of a special organ. As members of the order Pulmonobranchiatabreathers of atmospheric air-spin and use threads

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oftener than any other of the Gasteropoda, especially the aquatic members of the group, and as their method of using them differs from the Pectinibranchiata-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 upward 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 Limnæid issues from the egg 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 Physæ, 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.

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 supposed 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 property to a remarkable degree; each species produces mucus of a colour and consistency peculiar to itself, some species being provided with an important slime-gland near the tail. This property is essential to their well-being; having no sheltering shell, it serves to keep their body moist and cool in dry weather. Slugs often suspend themselves 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; but 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. Sphærium 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

spinning of which occupied the creature three nours. M. Bouchard-Chantereux 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.

4th. As a great part of the lifetime of the Limnæidæ, especially the Physæ, 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 Nudibranchs.*

The Sphæridæ, through their capability of climbing 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 molluscan 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 hypnorum.-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 Physæ and a Limnæa glabra upon a thread of the former at one time. Often, when two Physæ meet upon the same thread, they fight as only mollusks 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 gymnast that ever performed. I once saw one ascending, 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 creeping 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, which 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 in molluscan 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, performing such quick and curious evolutions, while seemingly floating in mid-water, is astonishing, even to the patient student of Nature's wonders.

Physa fontinalis stands next as a thread-spinner, using the thread in a similar manner, but not so often. Limnæa 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. peregra.-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 complanatus, P. spirorbis, P. contortus.-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.*

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PECTINIBRANCHIATA.

Bythinia tentaculata.-This snail suspends itself by a thread, after floating, which is usually attached to the surface of the water.

Rissoa parva is well known to conchologists as a thread-spinner. Mr. J. G. Jeffreys thus pleasantly speaks of it:-"Lying on a rock, by the brink of a seaweed-covered pool left by the receding tide, it is no less pleasant than curious to watch this active little creature go through its different exercises,creeping, floating, and spinning."

Several other species of Rissoæ spin threads, also Barleeia rubra, Eulima intermedia, Cerithium reticulatum, Cerithiopsis tubercularis, and Pleurotoma nebula. An account of their different modes of procedure will be found in Mr. Jeffreys's work, under their several headings.

Litiopa, a genus of small mollusks living on floating seaweed far from land, are said to use a mucous filament for the purpose of regaining their station, after having been swept off the weed.* If this be correct, we have a water-breathing mollusk using its thread as a means of ascent, after having spun it downwards, a circumstance I have not myself seen. My observation teaches me that these threads are not used by mollusks against the laws of gravitation.

With regard to the spinning of Neritina fluviatilis. This species is an inhabitant of running streams, and will not live long in confinement. Its structure renders it impossible for it to spin an upward thread, as the nature of its habitat alike precludes it, and as it could not float in running water, it could not therefore spin a downward thread, as obtains with other members of its order. While making these observations, I do not discredit Mr. Warington's statement, because, although the act of floating is not a normal one with the creature, it might have performed it as mollusks sometimes do,† when placed under circumstances which allow of it, albeit in their natural condition they could not possibly do it; and if it floated, there is no reason why it should not have spun a downward thread.

Having kept nearly every British species of the Limnæidæ in confinement on purpose to observe their habit of spinning, and not having seen some species use this means of locomotion at all, others seldom, and some often; some when young but less often as they grow older, and others all their lifetime, I have been led to advance a theory whereby to account for this varied use of these threads. To this end I have drawn up the following table. While writing it, I am sensible of its imperfections; but if it only serves as a nucleus to stimulate other ob

* Johnston, "Introduction to Conchology," p. 134. For an account of this habit in Trochus occidentalis, a deep-sea species, see Jeffreys, "Brit. Con.," vol. iii. pp. 335-6.

servers of the economy of these creatures to frame a more perfect one, I shall be the more satisfied with my attempt.

I must ask the reader to bear in mind that the time and opportunities at my command for observing their life and habits do not admit of my coming to the conclusion, that, because I have never seen a species spin a thread, therefore it does not do so. On the contrary, I believe that all the Limnæidæ use this method of travelling, more or less; and, doubtless, in their native habitat, when the eye of man is not present to pry into their secrets, these seemingly insignificant creatures perform these their appointed acts, while we, most wishing to see, see them not :

Planorbis lineatus.†-Inhabiting streams; could not spin a thread in its native habitat. I have not succeeded in keeping it alive long.

Planorbis nitidus,† P. nautileus,† P. albus,† P. glaber, P.vortex,† P. spirorbis,* P. contortus,* Limnæa truncatula.† Of these species, some spend their lives on vegetation near the surface of ponds or pools, and others inhabit shallow ponds or ditches, which sometimes become dry in summer; hence the necessity for using a thread does not often occur.

Planorbis carinatus,* P. complanatus.*— Living in the larger ponds and pools where the water is of considerable depth, this capability of thread-spinning often serves them to good purpose.

Physa hypnorum,* P. fontinalis,* Limnæa glabra.* -Inhabiting deep ditches, ponds, or pools, and fond of indulging in subaqueous excursions, the habit of spinning is essential to their mode of life.

Limnæa stagnalis,* L. palustris,* L. auricularia,† L. peregra, L. glutinosa, Planorbis corneus.— When full grown, these species, being much larger and stronger than any of the foregoing, are able to traverse more ground in a given time; hence they do not feel the necessity of using a thread so often as the smaller species.

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several contributors of this journal, I fancy some little interest has been excited to further investigations of these organs; for that reason I feel induced to give a brief sketch of another order of Homoptera, section Trissieræ, genus Aphrophora, species Aphrophora spumaria.

of tibia and tarsus being terminated with toothshaped spines (fig. 45).

The elytra when subjected to microscopical examination will be found very beautiful. The groundwork is seen to be made up of cellular tissue or

Fig. 43. Suctorial tube, lancets, &c., seen from beneath, X 120.

Fig. 4. Head and Awl-like antenna of Froghopper.

oval spots, arranged in irregular transverse rows. These spots are of uniform dimensions,* consisting of a centre, surrounded by a white circle, each spot being divided from its fellow by a space nearly equal

This insect, the "Frog-hopper," will probably be of more general interest, for, although so common, it is little known. Many persons no doubt have seen the frothy secretion upon the branches and leaves of shrubs during the summer months, and probably know that it is commonly designated cuckoo-spit, and this, perhaps, is the only idea of many concerning it. This secretion is effected on the larva leaving the egg. It covers itself with a froth, fixing its rostrum into the cellular tissue of the plant on which it is fixed; it draws up a sufficient quantity of sap to cover itself, pouring out a secretion from the organs placed at the terminal portion of the abdomen (this secretion is the spit); but a more careful observer will find, upon breaking it up, a small green larva, with yellow eyes; the insect thus protected passes through its larva of different stages until it arrives at maturity.

Upon subjecting the larva to the microscopic glass, the suctorial tube, lancets, &c., will be seen (fig. 43) composed of the labrum, which forms a jointed sheath for the slender trestle-like mandibles and maxillæ, and also a canal for the passage of the juices upon which the insects live.

In its mature state it is a dull, stone-bodied, inconspicuous insect with awl-like antennæ (fig. 44), from the last joint of which springs a trestle appendage. Posterior legs adapted for springing, the extremity

Fig. 45. Posterior leg and terminal claw.

to its own diameter; and, when viewed as an opaque object, stand out in relief. The females are furnished with a singular and beautiful apparatus (fig. 46), by which they are enabled to form excavations or grooves in the twigs or leaves of plants, for the purpose of depositing their eggs (which are large and few in number). It is analogous to the instrument possessed by the Tenthredo or Sawflies. On the under surface of the terminal segment of the abdomen, nearly at the extreme point, are seen a pair of valves or palpi, which form the sheath for the auger or boring instrument. Upon a casual view this auger appears like a denticulated arrowheaded spear, but on a more minute examination it is seen that what appears at first sight a single instrument, is made up of four distinct portions,. two deeply indented blades set back to back, and a middle support, in which they slide: the remaining two have their outer edges smooth, but the inner are cut into the most regular minute serrations. These facts are probably known to many; still there may be a few to whom this brief sketch may be in

* This is more discernible when the elytra is mounted on balsam.

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