the Muscle, in ichthyology; a genus of animals, belonging to the order of vermes tellacea. The animal is an acidia: the shell bivalve; often affixed to some substance by a beard; the hinge without a tooth, marked by a longitudinal hollow line. The most remarkable species are,

1. The rugosus, or rugged muscle, with a brittle shell, very rugged, and in shape very irregular; usually oblong, and round at the ends. Its length is near an inch; the colour whitish. It is always found lodged in limestone; the outside appears honey-combed; but the apertures are too small for the shell to pass through, without breaking into the cell they are lodged in. Multitudes are found in the same stone; but each has a separate apartment, with a different external spiral. 2. The edulis, or edible muscle, has a strong shell, slightly incurvated on one side, and angulated on the other. The end near the hinge is pointed; the other rounded. When the epidermis is taken off, it is of a deep blue colour. It is found in immense beds, both in deep water, and above low-water mark. The finest muscles are those called Hambleton hookers, from a village called Humbleton in that county. They are taken out of the sea, and placed in the river Wier, within reach of the tide, where they grow very fat and delicious. 3. The incurvatus, or crooked muscle, is very crooked on one side near the end; then greatly dilated, and covered with a thick rough epidermis. Within it has a violet tinge. It is found on the coast of Anglesea, near Prietholme; usually an inch and an half long. 4. The pellucidus, or pellucid muscle, hath a delicate transparent shell, most elegantly rayed lengthwise with purple and blue; like the former in shape, but more oval; commonly shorter than two inches. It is found in Anglesea, sometimes in oyster-beds, sometimes in trolling over flutty bottoms. 5. The umbilicatus, or umbilicated muscle, is nearly of an oval form, the length sometimes five inches. It is a rare and new species; sometimes dredged up off Prietholme island, Anglesea; discovered by the reverend Mr Hugh Davies. 6. The curtus, or short muscle, with a short, ventricole, obtuse shell, of a dirty yellow colour, length about an inch. 7. The modiolus, or great muscle, with a strong shell, blunted at the upper end; one side angulated near the middle; from thence dilating towards the end, which is rounded. It is the greatest of British muscles, being from six to seven inches in length; it lies at great depths; often seizes the baits of ground-lines, and is taken up with the hooks. 8. The cygnus, or swan muscle, with a thin brittle shell, very broad and convex, marked with concentric striae; attenuated towards one end, dilated towards the other; decorticated about the hinges; the colour a dull green; the length six inches, breadth three and a half; inhabits fresh waters. 9. The anatinus, or duck muscle, hath a shell more oblong and less convex than the last; is very brittle and semitransparent; the space round the hinges like the last; the length about five inches, breadth two and a quarter; inhabits fresh waters. Crows feed on these muscles, and also on different shell-fish. It is diverting to observe, that when the shell is too hard for their bills, they fly with it to a great height, drop the shell on a rock, and pick out the meat when the shell is fractured by the fall.

The common sea or edible muscle has, from its being always found fastened to the rocks, been supposed by many wholly incapable of progressive motion; but this is an erroneous opinion. It is a common practice in France, at such seasons of the year as do not afford sea enough to make salt, to throw the common sea-muscles, which the fishermen catch about the coasts, into the brine-pits. They have an opinion that this renders their flesh the more tender and delicate, as the rain which falls at these seasons makes the water of the pits much less salt than the common sea-water. The muscles are on this occasion thrown carelessly in, in several different parts of the pits; yet, at whatever distances they have been thrown in, the fishermen when they go to take them out, always find them in a clutter together; and as there is no current of water in these places, nor any other power of motion which can have brought the muscles together, it seems seems very evident that they must voluntarily have marched from the places where they were at first, to have met thus together. This progressive motion is wholly performed by means of what we call the tongue of the muscle, from its shape; but, from its use in this case, appears rather to merit the name of a leg, or an arm, as by laying hold of any distant substance, and then forcibly contracting itself again, it draws along the whole body of the fish; the same part, when it has moved the animal to a proper place, serves also to fix it there, being the organ by which it spins the threads which we call its beard, by which it is held to a rock, or to another muscle. The motion of the muscle, by means of this part, is just the same with that of a man laid flat on his belly, who would draw himself along by laying hold of any thing with one hand, and then drawing himself to it.

Muscles are well known to have a power of fastening themselves either to stones, or to one another's shells, in a very strong and firm manner; but the method of doing this was not well understood till the observations of Mr Reamur explained it.

Every one who opens and examines a common muscle, will find, that in the middle of the fish there is placed a little blackish or brownish body resembling a tongue. This in large muscles is near half an inch long, and a little more than a sixth of an inch in breadth, and is narrower at the origin than at the extremity: from the root of this tongue, or that part of it which is fastened to the body of the fish, there are produced a great number of threads, which, when fixed to any solid substance, hold the muscle firmly in its place: these threads are usually from an inch to two inches in length, and in thickness from that of a hair to that of a hog's bristle. They issue out of the shell in that part where it naturally opens, and fix themselves to any thing that lies in their way, to stones, to fragments of shells, or, which is the most common case, to the shells of other muscles; whence it happens that there are usually such large parcels of muscles found together. These threads are expanded on every side, and are usually very numerous, having been found issuing from one shell: they serve the office of so many cables; and, each pulling in its proper direction, they keep the muscle fixed against any force that can be offered from whatever part it come.

The filaments are well known to all who eat muscles, who ever carefully separate them under the name of the beard; and Mr Reamur has found, that while the animal is living in the sea, if they are all torn away by any accident, the creature has a power of substituting others in their room: he found, that if a quantity of muscles were detached from one another and put into a vessel of any kind, and in that plunged into the sea, they in a little time fastened themselves both to the sides of the vessel, and to one another's shells; the extremity of each thread seemed in this case to serve in the manner of a hand to seize upon any thing that it would fix to, and the other part which was slenderer and smaller to do the office of an arm in conducting it.

To know the manner of the muscles performing this operation, this diligent observer put some muscles into a vessel in his chamber, and covered them with sea-water; he there saw that they soon began to open their shells, and each put forth that little body before described by its resemblance to a tongue, and at the root of which these threads grow; they extended and shortened this part several times, and thrust it out every way, often giving it not less than two inches in length, and trying before, behind, and on every side with it, what were the proper places to fix their threads at: at the end of these trials they let it remain fixed for some time on the spot which they chose for that purpose, and then drawing it back into the shell with great quickness, it was easy to see that they were then fastened by one of these threads to the spot where it had before touched and remained fixed for a few minutes; and in repeating this workmanship the threads are increased in number one at every time, and being fixed in different places they sustain the fish at rest against any common force.

The several threads were found to be very different from one another; the new formed ones being ever whiter, more glossy, and more transparent than the others: and it appeared on a close examination, that it was not, as might have been most naturally supposed, the office of the tongue to convey the old threads one by one to the new places where they were now to be fixed, but that these in reality were now become useless; and that every thread we see now formed, is a new one made at this time; and in fine, that nature has given to some sea-fishes, as well as to many land-insects, a power of spinning those threads for their necessary uses; and that muscles and the like fish are under water, what caterpillars and spiders are at land. To be well assured of this, however, Mr Reamur cut off the beard or old threads of a muscle as close as he could, without injuring the part; and the proof of the opinion of their spinning new ones at pleasure was now brought to this easy trial, whether these muscles, so deprived of their old ones, could fix themselves as soon as others which were possessed of theirs, and could throw out their threads to as considerable distances. The experiment proved the truth of the conjecture; for those whose beards or old threads were cut off, fixed themselves as soon as those in which they were left, and spread their threads to as great a distance every way.

When the mechanism of this manufacture was thus far understood, it became a natural desire to inquire into the nature of the part by which it was performed: this has hitherto been mentioned under the name of the tongue, from its shape; but it is truly the arm of the fish; and whenever it happens to be loosened from its company, or fixed in a wrong place, it serves the animal to drag its whole body shell and all along, and to perform its several motions. It fixes itself to some solid body; and then strongly contracting its length, the whole fish must necessarily follow it, and be pulled toward the place where it is fixed. This is an use, however, that this part is so rarely put to, that it is not properly to be esteemed a leg or an arm, for this; but, according to its more frequent employment, may much better be denominated the organ by which the threads are spun.

Though this body is flat in the manner of a tongue for the greater part of its length, it is however rounded or cylindric about the base or insertion, and it is much smaller there than in any other part: there are several muscular ligaments fastened to it about the root or base, which hold it firmly against the middle of the back of the shell; of these ligaments there are four which are particularly observable, and which serve to move the body in any direction. There runs all along this body a slit or crack, which pierces very deeply into its substance, and divides it as it were into two longitudinal sections; this is properly a canal, and along this is thrown the liquor which serves to form the threads; and it is in this canal or slit that these threads are moulded into their form. Externally, this appears only a small crack or slit, because the two fleshy sections of the parts almost meet and cover it, but it is rounded and deep within, and is surrounded with circular fibres. This canal is carried regularly on from the tip of the tongue, as it is called, to its base, where it becomes cylindric; the cylinder in this part being no other than a close tube or pipe, in which this open canal terminates. The cylindric tube contains a round along body, of the nature of the threads, except that it is much larger; and from the extremity of this all the threads are produced, this serving as a great cable to which all the other little cordages differ towards different parts are fixed. The tube or pipe in which this large thread is lodged, seems the reservoir of the liquor of which the other threads are formed; all its internal surface being furnished with glands for its secretion.

The muscle, like many other sea-fishes, abounds in this liquor; and if at any time one touch with a finger the base of this spinning organ, one draws away with it a viscous liquor in form of several threads, like those of the caterpillar, spider, and the other spinning land-animals. The threads fix themselves with equal ease to the most smooth and glossy, as to rougher bodies; if the muscles are kept in glass jars of sea-water, they as firmly fasten themselves to the glass as to any other body.

Muscles, be they ever so young, have this property of spinning; and by this means they fasten themselves in vast numbers to anything which they find in the sea. Mr Reaumur has seen them when as small as millet seeds, spin plentifully; tho' their threads, proportioned to their own weight, are much finer and smaller than those of larger muscles.

It is a question yet undetermined, whether the muscle has a power of breaking or otherwise getting rid of its threads, in order to its removing from the place where it is once fixed; but it appears probable that they have not, and that they must remain where they have once fastened themselves, tho' their destruction be the consequence of it. Mr Reaumur tried this experiment in his jars; when they had well fixed themselves to the sides of them, he poured off part of the salt-water, so that it became the interest of the fish to leave their hold and go lower down, but they seemed to have no power to effect this.

The common muscle affords the curious observer a very pleasing object of examination by the microscope. The transparent membrane, which immediately appears on opening the shell, shows the circulation of the blood for a long time together through an amazing number of vessels. And Mr Lewenhoek, in several which he dissected, discovered numbers of eggs or embryo muscles in the ovarium, appearing as plainly as if he had seen them by the naked eye, and all lying with their sharp ends fastened to the string of vessels by which they receive nourishment. The minute eggs, or embryos, are by the parent placed in due order, and in a very close arrangement on the outside of the shell, where, by means of a gluey matter, they adhere very fast, and continually increase in size and strength, till becoming perfect muscles, they fall off and shift for themselves, leaving the holes where they were placed, behind them.

This abundance of muscle-shells very plainly show when examined by the microscope, and sometimes they are in the number of 2000 or 3000 on one shell; but it is not certain that these have been all fixed there by the muscle within; for these fish usually lying in great numbers near one another, the embryos of one are often affixed to the shell of another. The fringed edge of the muscle, which Lewenhoek calls the beard, has in every minute part of it such variety of motions as is inconceivable; for being composed of length fibres, each fibre has on both sides a vast many moving particles.

From the common muscles abundance of small pearls, called seed-pearls, were, till of late, procured for medical purposes; but they are now disused since it became generally known, that the cheaper taffeta-powders were equally efficacious with these. Pearls are also found in the two last species.—Muscles sometimes disagree with those who eat them, bringing on swellings, difficulty of breathing, blotches, and sometimes even death. The cure is oil mixed with salt water.