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from many familiar trials of its strength, how firmly the animal doth adhere. Reaumur ascertained that a weight of twenty-eight or thirty pounds was required to overcome the force of it. This astonishing power in so small and hebetous an animal does not depend on the muscularity of the foot, nor on any mechanical engraining of its surface with the pores of the stone, nor on any vacuum produced under it; for Reaumur disproved all these explanations by some decisive experiments. He cut the animal from top to bottom in two halves as it stood fixed perpendicularly on the rock, and he made other deep incisions in a horizontal direction, destroying in this manner all the muscular power of its base, and all supposable vacuity between it and the stone, but the adhesion continued as firm as before the experiment. Even the death of the limpet does not destroy the cohesion. This entirely depends on a glue, or kind of paste, which, although invisible, produces a very considerable effect. If, after having detached a Patella, the finger is applied to the foot of the animal or to the spot on which it rested, the finger will be held there by a very sensible resistance, although no glue is perceptible. And it is remarkable, that if the spot is now moistened with a little water, or if the base of the animal is cut, and the water contained in it allowed to flow over the spot, no further adhesion will occur on the application of the finger, the glue has been dissolved. It is Nature's solvent by which the animal loosens its own connection to the rock. When the storm rages, or when an enemy is abroad, it glues itself firmly to its rest; but when the danger has passed, to free itself from this enforced constraint, a little water is pressed from the foot, the cement is weakened and dissolved, and it is at liberty to raise itself and be at large. The fluid of cementation, as well as the watery solvent, are secreted in an infinity of miliary glands, with which the foot is, as it were, shagreened;* and as the limpet cannot supply the secretion as fast as this can be exhausted, you may destroy the animal's capacity of fixation by detaching it forcibly two or three times in succession.

Of the habits of the Patella, so far as they concern us at present, Mr. Lukis, of Guernsey, gives the following account:- "The locomotion of the limpet may be ascer

* Adanson took these glands for little suckers, to whose combined action he attributed principally the animal's adhesion.-Seneg. p. 31.

In the Onchidium peronii, in some species of Doris, and particularly in the Onchidore, the foot is sprinkled over with a great number of vesicles or vesicular tubercles.-BLAINVILLE in Journ. de Physique, lxxxv, p. 439.

tained by marking one individual, to avoid mistake, and then observing its cautious roaming, and regular return to its favourite place of rest, where the shell will be found exactly to correspond with the surface of the rock to which it is attached. Here it will rest, or sleep, and only relax its strong adhesion to the rock when the muscular fibre becomes exhausted by long contraction, in which state a sudden blow horizontally given will easily displace it. A fact known to the fishermen and poor, who use them for food, is, that they are more easily collected in the night-time than in the day. May not this be the period of roaming for food, as well as when covered by the tide?

"The march of the limpet is slow and formal; and, whenever the cupping process is renewed, the posterior end of the shell is brought in contact with the rock, which, if of a soft nature, will receive the impressions of its denticulations." The track of an individual, placed under surveillance, was thus made visible over a space of several yards, possessing the same regularity and disposition, and was further remarkable for the constant revolution on its left.

"The tracks of the limpet on granite and other hard rocks present at first sight the same appearances; but on a closer examination they are found to differ." When first observed, in 1829, a large portion of a fine-grained sienitic rock was traced over by these shells; the remainder was plain, and appeared varnished with a thin coating of some kind of fucus, without any markings upon its surface. "As no Patellæ were at first discovered, and the isolated situation of the rock prevented any from reaching it, I was at a loss to explain these appearances; but after some search, a fissure was found at the north end, where five or six limpets had fixed themselves, each having a direct road leading to their pasturage-ground. By the help of a glass, the markings visible on the rock were discovered to be the remains of the above fucus, which had been eaten through or trodden down by these animals in their excursions, and which retained the indentures of their shells. The edge of the vegetable surface was then examined, and found to be nibbled in a circular manner, resembling the anterior margin of the shells."*

* Mag. Nat. Hist. iv. 347.

149

LETTER X.

THE BORING MOLLUSCA AND NEST-BUILDERS.

I MUST now beat back a little, having been led, almost inadvertently, to pass unnoticed a tribe nearly allied to the burrowers in sand and gravel, but of deeper interest. The tribe in question are almost exclusively Bivalves, and, to secure themselves, they excavate their cells in solid bodies, -in wood, hardened clay, and harder rocks,-whence they cannot again issue or be removed; their house during life, and after death, their grave. The Teredo bores his long tortuous cell in wood; the Pholades construct their more capacious dwellings in wood and in clay; the Lithodomi and Saxicavæ excavate limestone rocks, coral-reefs, and the thick shells of other mollusks; while the Fistulanæ and Clavagellæ are said to bore indifferently into sand, wood, rocks, and into shells. In general, each species confines itself to one kind of substance, but this is not always the case. Olivi says, that he has twice seen Pholades in a piece of compact lava;* the common European species of that family are found as often in timber as in clay, and some of them perforate likewise calcareous rocks. Montagu tells us, he had specimens of Gastrochæna modiolina in common limestone, in fluor, and in granite; † and Dr. Pulteney speaks of Venerupis irus as being plentiful on the Dorset coast in clay as well as in limestone.-They are to be found on all shores, from Greenland to the furthest Ind. Within the tropics, however, they are most abundant, and of the largest size; but the station most celebrated in history is European, viz. in the Bay of Naples, near to Pozzuolo, where a colony of Lithodomi § had settled themselves in the pillars of the temple of Jupiter Serapis during the period

*There is probably some error in this observation, so far as it would lead to the inference that the Pholas had bored into the lava. Spallanzani, who tells us that he had given particular attention to the subject, never observed the lithophagous mollusca to "make their lodgments but in calcareous stones."-Trav. in the Two Sicilies, i. 178.

+ Test. Brit. Supp. 25.

Ibid. 109.

§ Bohadtch says they are Pholades which have made these excavations.— An. Marin. 154. But this is a mere licence of nomenclature: the species is the Mytilus lithophagus, Linn. Modiola lithophaga, Lum.

of its submersion. At the height of ten feet above the base of the three standing pillars which remain, and in a position exactly corresponding in all, is a zone of six feet in height,* where the marble has been scooped into cells by these mollusca (Fig. 26). The holes are to the depth of four

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inches; and it is observed that the nodules of quartz and feldspar, which sometimes occur in the hard limestone of the pillars, are untouched. In what manner this temple was submerged and again left dry has much puzzled and perplexed philosophers, and the discussion is, fortunately, beyond our province; but it becomes us to inquire by what means shell-fish make these holes, for which, apparently, they are most unfit.

The point has been much debated, and it seemed so hard to solve, that Rondeletius saw nothing for it but to suppose that the sea-water, lodging in the rocks, was itself transformed into Pholades and other saxicavous mollusca; and other philosophers, as Mr. Bingley good-naturedly calls them, were driven to the belief that they entered the rock while it was yet in a soft state, which afterwards hardened by degrees around them. Two explanations of the process

*Spallanzani says that the height from the ground is about nine feet, and that the perforated band is only "about two feet in breadth." Two species of Mytilus or Modiola and "other lithophagous worms "have worked to produce it.-Trav. in the Two Sicilies, i. 84, 85.

had for long divided less imaginative naturalists,-the one, that the creatures bore by the aid of a solvent liquor which they excrete; the other, that they do so by processes, or hard portions of the shell, worked by its semi-rotatory motions, and regulated by appropriate muscles. Of late, other two theories have been propounded: the borings have been ascribed, by an ingenious author, to the action of currents of water, directed against the parts to be worn away, by the ceaseless play of cilia on the animal's body, these currents acting not so much by their force, as by their constant and long-continued impulse, just as the drop from the eave will in time wear a basin in the stone floor underneath. The other theory ascribes the whole works of the whole tribe to the animal itself operating on the wood or rock with an organ fit and fitted for the purpose.

The mechanical theory seems to have been suggested, in the first instance, by a false view of the valves of the Teredo, which their form misled the earlier naturalists to believe were the teeth of the animal, and that with them the animal eat its way into the wood.* When this mistake of office was discovered, the organs were construed to be augers; and the other boring mollusks, it was affirmed, had organs adapted for a similar purpose either in the spinous processes or in harder and thicker margins in front, or, as in the instance of the Lithodomus, in the shape of the shell itself, which needed only to be put into a rotatory and forward motion by the muscular efforts of the mollusk. The fact of there being such a rotation of the shell was deemed to be proved by the circular striæ which some naturalists had observed on the walls of the cells of our common Pholades.

The chemists, on their part, were driven to a hypothetical acid by the many objections which seemed to render any mechanical explanation untenable. There was no proportion between the creature's physical powers and the results produced; the substance operated upon was, in many cases, harder than the shell, and more likely to wear away its processes and asperities than itself to be perforated; and yet the shell remained intact, and there were no appearances on its parts or surface to indicate that it had been used either as a rasping or boring instrument, the very skin covering the valves remaining uninjured. It was even said, that the form and size of the cell made a rotatory motion of the animal in it impossible, as surely was the case

* Home's Comp. Anat. i. 377. Delle Chiaie defines the Teredo-“ Animal-anterius maxillis lignum terebrans.”—Anim. Nap. s. Vert. iv. 32.

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