unnecessary; and having an extensive fulcrum, this species can therefore excavate in soft substances with as much facility as Pholas." *

There are no borers amongs the tunicated, brachiopod, or cephalopod mollusca; but a few of the Gasteropods have the same power, and it has been attributed to others, on rather uncertain grounds. The Limpet (Patella vulgata) seems to be in the habit of hollowing out a space in the site on which it has settled, answering to the size and shape of the rim of the shell; and it does this probably with the intention of obtaining a stronger seat or hold with the expenditure of less muscular power, or rather, with the relaxation of all muscular contraction, so that the shell may be elevated sufficiently to admit the influx of water around the branchia without any danger of the animal being driven from its settlement. The excavation varies in depth, not according to the chemical composition, but according to the softness of the site,-from a line to half an inch; but the shell is never buried in it. Mr. J. E. Gray, who first called particular attention to this peculiarity in the limpet's habits, explained the operation as the result of a solvent fluid, excreted from the sole of the foot, of which, however, no evidence was produced. De Montfort came near the truth. Some Patellæ, he says, when in a state of repose have a peristaltic motion in the foot, which slowly hollows out the stone on which they rest by friction alone, and these species are sedentary. "By friction alone" the operation is done, and the foot is armed with hard crystalline siliceous spicula, similar in all respects to those of the perforating instrument of the Pholas, and replaced by new ones as often as the friction has rubbed off the asperities of the old.

The questionable Gasteropod borers are our common snails. I shall give you all the evidence I possess, and, curiously enough, the question has become connected with some interesting geological phenomena, which lie, however, beyond our demesne. "On the east side of Whelpington," says the Rev. Mr. Hodgson, in his " History of Northumberland,' a stratum of limestone is here and there seen in grey projecting masses, the under-surface of which is bored upwards into cylindrical holes, which are from a line to four inches deep, and tenanted, especially in winter, by the banded and yellow varieties of the Helix nemoralis. The Limax, while it occupies these cavities during the summer, has its fleshy

longitudinal disk protruded out of the shell, and coiled nearly into a circle on the surface of the stone, the summit of its shell hanging downwards; and in this position it probably elaborates its den in the same manner that some of the Pholades work their way into clay and wood, or, by a slow but constant process, sink and enlarge their cells in the hardest stones." In the subsequent page, the Reverend annalist attributes the same property to Helix (Limnæus) putris, and makes it the agent of holes in the bed of the little river Wansbeck. The proof, you will observe, here led for the Helices being the operators is very inconclusive; and I shall leave the following, made by a more scientific observer, to your own judgment:-"During the meeting of the Geological Society of France, at Boulogne, in September, 1839, Dr. Buckland's attention was called by Mr. Greenough to a congeries of peculiar hollows on the under-surface of a ledge of carboniferous limestone rocks. They resembled, at first sight, the excavations made by Pholades, but as he found in them a large number of the shells of Helix aspersa, he inferred that the cavities had been formed by snails, and that probably many generations had contributed to produce

them.

"A few years since (viz., previous to 1841), the Rev. N. Stapleton informed the author that he had discovered at Tenby, in the carboniferous limestone on which the ruins of the castle stand, perforations of Pholades thirty or forty feet above high-water level; but having recently examined the spot, Dr. Buckland ascertained that these excavations were the work of the same species of Helix as that which had formed the cavities in the limestone near Boulogne, and he found within them specimens of the dead shells as well as of the living. The mode of operation by which the excavations were made, he conceives, is the same as that by which the Common Limpet (Patella vulgata) corrodes a socket in calcareous rocks, and he is of opinion that the corrosion is due to the action of some acid secreted from the body of the limpet or helix." A little afterwards the celebrated professor observes, that the snails "could find shelter only on the margin and lower surface of the projecting rock, and the irregular form of the confluent cavities correspond with that of the clusters of snails in their ordinary habitat and hybernation; and if to those reasons be added, the fact of finding both living and dead shells in the excavations, the evidence, the author conceives, is decisive as to the agency of snails in

Hist. North. part ii. vol. i. 193.

producing the phenomena under consideration."* I beg to put in a demurrer to this conclusion, until more decisive proof is given me in the detection of an organ fitted to make the snail a boring operative.

A very few mollusks not endowed with the mechanical power to bore and tunnel, and yet, as it were, conscious that some extraneous covering would be useful to them, set about to provide it each after its fashion. Thus the Phorus, which is a genus of nomade Gasteropods, gathers together the dead and living shells of other species, and intermingling them with pieces of broken coral and small stones, glues them to the outside of its shell so as to hide its true features and make it pass as a dead mass of inorganic matter; or, as when the cemented shells are of the turreted kind, it may frighten thus away an enemy, for the shells, pointing in all directions, present to view an armature of strong spikes, on which even a hungry fish might hesitate to feed. Some Modiolæ cover themselves with a hairy vestment made of the threads of their byssus;t and one native Crenella (C. discors), "forms for itself a kind of nest or case by stitching together the small sea-weeds or corallines with its byssal threads; "while another (C. marmorata) digs deep into the leathery coat of Ascidia, and very effectively hides itself there. The unmoving Gastrochæna modiolina is found on the Guernsey shores, living in the crevices of rocks amidst the débris of madrepores, and of shells and gravel; and with this material it puts together a sort of nest or chamber that resembles a Florence flask, and completely encases the shell. The outside of the nest is rough, but the inside is smooth, and consists of thin layers of a calcareous secretion applied by the animal, which leaves open the neck of the nest whence it protrudes the tubes of respiration and of effete matters. The animal can, of course, enlarge the nest to suit its growth, and it can also prolong the neck so as to keep it above any overlying madrepores. § The Lima, whose active habits I have had already occasion to notice, is also occasionally a nest

* Ann. and Mag. N. Hist. viii. 459.

+ Philippi thus describes the nest of his Modiola vestita: "Involucrum mirabile sacci instar totam testam occultans, intus e tomento filorum cinereorum, extus e lapillis, conchyliorum fragmentis et similibus compositum est, et cum parte postica cohæret, a cujus filis ex parte ortum videtur. Byssum nullum inveni, eumque fugacem, a filis tenuissimis contextum fuisse saccoque forte altera ex parte originem dedisse puto.”—Mollusc. Sicil. ii.

51.

Alder in Trans. Tynes. Nat. Club, i. 175.

§ Loudon's Mag. N. Hist., vi. 404. All Gastrochænæ make similar bottle-shaped cases. See Forbes and Hanley's Brit. Mollusca, i. 133.

builder; and I must not injure the interest of the following account of it by any curtailment. With a party of friends the Rev. David Landsborough dredged Lamlash Bay, on the 4th of June, 1846, and he says:-"The most interesting, though not the rarest thing we got, was Lima hyans of continental writers, Lima tenera of Turton. I had before this some specimens of this pretty bivalve, and I had admired the beauty and elegance of the shell, but hitherto I had been unacquainted with the life and manners of its inhabitant. Mr. and Miss Alder had got it in the same kind of coral at Rothesay, so that when Miss Alder got a cluster of the coral cohering in a mass, she said, "O, here is the Lima's nest!" and breaking it up, the Lima was found snug in the middle of it. The coral nest is curiously constructed, and remarkably well fitted to be a safe residence for this beautiful animal. The fragile shell does not nearly cover the mollusk-the most delicate part of it, a beautiful orange fringe-work, being altogether outside of the shell. Had it no extra protection the half-exposed animal would be a tempting mouthful-quite a bonne-bouche to some prowling haddock or whiting; but He who tempers the wind to the shorn lamb, teaches this little creature, which he has so elegantly formed, curious arts of self-preservation. It is not contented with hiding itself among the loose coral, for the first rude wave might lay it naked and bare. It becomes a marine-mason, and builds a house or nest. It chooses to dwell in a coral grotto. But in constructing this grotto it shows that it is not only a mason but a rope-spinner, and a tapestry-weaver, and a plasterer. Were it merely a mason it would be no easy matter to cause the polymorphous coral to cohere. Cordage, then, is necessary to bind together the angular fragments of the coral, and this cordage it spins; but it spins it as one of the secrets of the deep. Somehow or another, though it has no hand, it contrives to intertwine this yarn which it has formed among the numerous bits of coral so as firmly to bind a handful of it together. Externally, this habitation is rough, and therefore better fitted to elude or to ward off enemies; but though rough externally, within all is smooth and lubricous, for the fine yarn is woven into a lining of tapestry, and the interstices are filled up with fine slime, so that it is smooth as plasterwork, not unlike the patent Intonaco of my excellent, ingenious friend, Mrs. Marshall. Not being intended, however, like her valuable composition, to keep out damp or to bid defiance to fire, while the intertwining cordage keeps the coral walls together, the fine tapestry mixed with smooth and

moist plaster, hides all asperities, so that there is nothing to injure the delicate appendages of the enclosed animal. Tapestry, as a covering for walls, was once the proud and costly ornament of royal apartments; but ancient though the art was, I shall answer for it that our little marine artisan took no hint from the Gobelins, nor from the workmen of Arras, nor from those of Athens, nor even from the earliest tapissiers of the East. I doubt not, that from the time Noah's ark rested on the mountain of Ararat, the forefathers of these beautiful little Limas have been constructing their coral cottages, and lining them with well-wrought tapestry in the peaceful bay of Lamlash.

"When the Lima is taken out of its nest, and put into a jar of sea-water, it is one of the most beautiful marine animals you can look upon. The shell is beautiful; the body of the animal within the shell is beautiful; and the orange fringe-work outside of the shell is highly ornamental. Instead of being sluggish, it swims about with great vigour. Its mode of swimming is the same as that of the scallop. It opens its valves, and suddenly shutting them, expels the water, so that it is impelled onwards or upwards; and when the impulse thus given is spent, it repeats the operation, and thus moves on by a succession of jumps. moving through the water in this way, the reddish fringework is like the tail of a fiery comet. The filaments of

When

the fringe are probably useful in catching its prey. They are very easily broken off, and it is remarkable that they seem to live for many hours after they are detached from the body, twisting themselves like so many worms."

*

I shall conclude this long letter with a table, from the study of which you may be enabled to methodize the information I have endeavoured to give you relative to the locomotion of the mollusca; and it will also exhibit the analogies, which, in this view, exist between the various orders and families.

* Excursions to Arran, p. 319.