or SEA-PEN, in natural history, a genus of zoophyte, which, though it swims about freely in the sea, approaches near to the gorgonia. This genus hath a bone along the middle of the inside, which is its chief support; and this bone receives the supply of its osseous matter by the same Pennatula polype mouths that furnish it with nourishment. Linnaeus reckons seven species. The name zoophytes under which this genus is ranked, is well known signifies, that the creature partakes both of the animal and vegetable nature; but some have supposed it to be nothing more but a fucus or sea plant. It is certainly an animal, however, and as such is free or locomotive. Its body generally expands into processes on the upper parts, and these processes or branches are furnished with rows of tubular denticles: they have a polype head proceeding from each tube.

The sea-pen is not a coralline, but distinguished from it by this specific difference, corals, corallines, alcyonaria, and all that order of beings, adhere firmly by their bases to submarine substances, but the sea-pen either swims about in the water or floats upon the surface.

The Honourable Dr Coote Molesworth lately sent one of these animals to the ingenious Mr Ellis, the author of many curious papers on the nature of corallines, which was taken in a trawl in 72 fathoms water, near the harbour of Bretz in France: the same species are frequently found in the ocean from the coast of Norway to the Mediterranean sea, sometimes at considerable depths, and sometimes floating on the surface. Mr Ellis's description of that sent him by Dr Molesworth is as follows:

Its general appearance greatly resembles that of a quill feather of a bird's wing (see Plate CCCLXXXVIII, fig. 1); it is about four inches long, and of a reddish colour; along the back there is a groove from the quill part to the extremity of the feathered part, as there is in a pen; the feathered part consists of fins proceeding from the stem, as expressed in the figure. The fins move the animal backward and forward in the water, and are furnished with suckers or mouths armed with filaments, which appear magnified as fig. 2. There is no perforation at the bottom, and therefore Mr Ellis of opinion, that the exuvia of the animals upon which it feeds are discharged by the same apertures at which the food is taken in; and in this it is not singular, Nature having observed the same economy in the Greenland polype, described by Mr Ellis in his Essay on Corallines. Each sucker has eight filaments, which are protruded when prey is to be caught; but at other times they are drawn back into their cases, which are furnished at the end with ficulae, that close together round the entrance, and defend this tender part from external injuries.

Dr Bohadich of Prague had an opportunity of observing one of these animals alive in the water, and he gives the following account of what he saw: "A portion of the stem contracted, and became of a strong purple colour, so as to have the appearance of a ligature round it; this apparent ligature, or zone, moved upwards and downwards successively through the whole length of the stem, as well the feathered as the naked part; it began at the bottom, and moving upwards to the other extremity, it there disappeared, and at the same instant appeared again at the bottom, and attended as before; but as it ascended through the feathered or pinnated part, it became paler. When this zone is much constricted, the trunk above it swells, and acquires the form of an onion; the con- stricture of the trunk gives the colour to the zone, for the intermediate parts are paler in proportion as the zone becomes deeper. The end of the naked trunk is sometimes curved like a hook; and at its extremity there is a finus or chink, which grows deeper while the purple ring is ascending, and shallower as it is coming down. The fins have four motions, upward and downward, and backward and forward, from right to left, and from left to right. The fleshy filaments, or claws, move in all directions; and with the cylindrical part from which they proceed are sometimes protruded from the fins, and sometimes hidden with them.

Upon dissecting this animal the following phenomena were discovered. When the trunk was opened lengthwise, a fleshy liquor flowed out of it, so viscid as to hang down an inch. The whole trunk of the stem was found to be hollow, the outward membrane being very strong, and about a tenth part of an inch thick: within this membrane appeared another much thinner; and between these two membranes, in the pinnated part of the trunk, innumerable little yellowish eggs, about the size of a white poppy-seed, were seen floating in a whitish liquor; about three parts of the cavity within the inner membrane is filled by a kind of yellowish bone: this bone is about two inches and an half long, and one twentieth of an inch thick; in the middle it is four square, but towards the ends it grows round and very taper, that end being finest which is next the pinnated part of the trunk. This bone is covered in its whole length with a clear yellowish skin, which at each end runs out into a ligament; one is inserted in the top of the pinnated trunk, and the other in the top of the naked trunk: by the help of the upper ligament the end of the bone is either bent into an arch, or disposed in a straight line. The fins are composed of two skins; the outward one is strong and leathery, and covered over with an infinite number of crimson streaks; the inner skin is thin and transparent: the suckers are also in the same manner composed of two skins, but the outward skin is something softer. Both the fins and suckers are hollow, so that the cavity of the suckers may communicate with those of the fins, as the cavity of the fins does with that of the trunk.

Dr Shaw, in the History of Algiers, says, that these animals are so luminous in the water, that in the night the fishermen discover fishes swimming about in various depths of the sea by the light they give: From this extraordinary quality Linnæus calls this species of the sea-pen, pennatula phosphorea, and remarks, after giving the synonyms of other authors, Habitat in oceano fundum illuminans.

There are other kinds of sea pens, or species of this animal, which have not a resemblance to a pen. There is the kidney-shaped sea-pen (see fig. 3.), the feather of the peacock fish (see fig. 4.), the pennatula filosa of Linnæus (see fig. 5.), his pennatula sagitta (see fig. 6.), his pennatula mirabilis (see fig. 7.), and the finger-shaped sea-pen (see fig. 8.). The kidney-shaped sea-pen was discovered some time ago on the coast of South Carolina, and sent to Mr Ellis by John Gregg, Esq.; of Charlestown. It is of a fine purple colour; the kidney part is about an inch from end to end, and about half an inch wide in the narrowest part; a tail proceeds from the middle of the body, which is roundish, and about an inch long; is also full of rings like an earth-worm, and along the middle both of the upper and under part of it there is a small groove which runs from one end to the other, but there is no perforation at either extremity. The upper part of the body is convex, and about an inch thick; the whole surface is covered with small yellow flary openings, through which little suckers are protruded, each furnished with six tentacula, or filaments, like what are observed on some corals; the under part of the body is quite flat, and is full of ramifications of fleshy fibres, which proceeding from the insertion of the tail, as a common centre, branch out so as to communicate with the flary openings on the exterior edge and upper surface of the animal. Of all the pennatula yet known the feather shaped one, or as it is called the silver sea-pen (see fig. 1), is the largest as well as the most precious in its appearance. It is of a beautiful silvery white, elegantly striated on each of the feather-like processes with lines or streaks of the deepest black. It is very rare, and is a native of the Indian seas. There is a very fine specimen of this species in the British Museum.