If you were to go on a long voyage, there is nothing that you would be much more pleased with, than the occasional bright appearance of the sea at night. You might see this in all latitudes, but more frequently in the tropical, than in colder climates. It occurs in a variety of modes, and, as it seems, arises from several different causes. Sometimes the wake of the vessel looks like a waving line of beautiful silver light; sometimes the surface will be studded with spots of bright light, about as large as your heads; not unfrequently you may see large islands of light; and now and then a shoal of Albicores, or some other fish, will pass the vessel, shaking sparks of dazzling brightness from their fins in all directions.
It has been supposed by some that the brightness in the track of a vessel, and that which sparkles and flashes in the spray on the tops of the waves, is electrical light: whether it is so or not, I cannot tell you.
The effect is, no doubt, sometimes produced by decayed animal and vegetable bodies, which appear bright in the dark. I dare say you have seen the bodies of dead fish, and the surface of rotten wood have this appearance. The particles in which the brightness exists are so small, that you cannot see them, for, if you touch the luminous body, particularly if it is a stinking fish, your fingers will immediately appear bright in the dark, and when you look at them in the light, you will not be able to see anything on the surface of the skin.
But the most wonderful and extensive cause of all, is the existence of countless myriads of fish and insects, which are supplied with a fluid substance that oozes out of their bodies, and shines with a greenish light upon their surface. Some of them may easily be taken, by dropping over a net where the luminous spots appear. A few of them are small shell-fish, of the crab or lobster kind. However, the greater part are an immense family of creatures called Acalephæ, or sea nettles. The word Acalepha is the Greek for a sting nettle, and they are so called for a reason which I will tell you presently.
There are a great many different forms of them, but they mostly agree in having their bodies shaped something like an umbrella, with long filaments hanging down from them. This is a picture of one of the kind called Berenice. It takes its name from the resemblance of its filaments, to the long hair of a lady. Berenice was the wife of a warrior, who made a vow to sacrifice the hair of her head, which was singularly beautiful, to Venus, if her husband returned from some successful exploit in which he was engaged. He did so, and the lady accordingly hung up her hair in the temple of Venus, and thereby gained the honor of giving her name to a constellation called the hair of Berenice, and the curious animal figured in the cut.
The substance of the body is a mere mass of jelly, mostly quite transparent, but sometimes tinged with blue or green; and when deprived of life it becomes merely a salt liquid, and an extremely thin skin, weighing only a few grains.
The mouth is underneath, in amongst the filaments, which seem to be placed where they are for the purpose of entangling small fish and insects, to hold them till the Acalepha can swallow them. A very great number of them are of a kind called Medusa, from the resemblance of their filaments to the snakes, which were said to take the place of hair upon the head of Medusa, one of the Furies. One or two sorts have a crest which they erect as a sail, and thus move along before the wind, on the surface of the sea, in calm weather. I will show you the picture of one of these called the Physalia.
