Bibra, a Brazilian navigator, having caught six Pyrosoma, employed them to light up his cabin. The light produced by these little creatures was so bright, that he could read to one of his friends the description he had written of these his living torches.
Three species of Pyrosoma are known; namely, P. elegans, two or three inches in length, which inhabits the Mediterranean; P. giganteum, which is found in the same sea. It is a long bluish cylinder, bristling with tubercles, each of which is the abode of an animal, a citizen of this moving republic, and is attached to its colleagues by means of its gelatinous envelope: an alliance imposed by inexorable Nature—a forced species of socialism.
The third species, P. atlanticum, was discovered by Péron and Lesueur in the Equatorial seas.
These curious Ascidians are so created in rings as to constitute a long fine cylindrical tube, closed at one end and open at the other. By the contraction and dilatation of the mass of beings, this great cylinder swims slowly through the open sea, lighting up the ocean with its phosphorescent light, shining through the water like a glowing fire. Mr. Bennet thus describes one of these pelagic appearances: "On the 8th of June, being then in lat. 30° S. and 27° 5' W. long., having fine weather and a fresh south-easterly trade-wind, and the thermometer ranging from 78° to 84°, late at night the mate of the watch called me to witness a very unusual appearance in the water. This was a broad and extensive sheet of phosphorescence extending from east to west as far as the eye could reach. I immediately cast the towing-net over the stern of the ship, which soon cleaved through the brilliant mass, the disturbance causing strong flashes of light to be emitted, and the shoal, judging from the time the vessel took in passing through the mass, may have been a mile in breadth. On taking in the towing-net, it was found half filled with Pyrosoma atlanticum, which shone with a beautiful pale greenish light. After the mass had been passed through by the ship, the light was still seen astern, until it became invisible in the distance, and the ocean became hidden in the darkness as before this took place.
"The second occasion of my meeting these creatures was in a high latitude, and during the winter season. It was on the 19th of August, the weather dark and gloomy, with light breezes from north-northeast, in lat. 40° 30' S., and 138° 3' E. long., at the western entrance to Bass's Straits, and about 8 o'clock p.m., when the ship's wake was perceived to be luminous, while scintillations of the same light were abundant all round. To ascertain the cause, I threw the towing-net overboard, and in twenty minutes succeeded in capturing several Pyrosoma, which gave out their usual pale green light; and it was, no doubt, detached groups of these animals which were the occasion of the light in question. The beautiful light given out by these molluscans soon ceased to be seen; but by moving them about it could be reproduced for some length of time after. The luminosity of the water gradually decreased during the night, and toward morning was no longer seen."
The genus Salpa forms another interesting group of Tunicata.
The Biphora or Salpa (Fig. 126) are long transparent threads of the more delicate tissues, composed of rows of individuals placed side by side, and grafted, as it were, transversely: ribbons, in which each animal is grafted end on end to its sister: double parallel chains of social creatures, sometimes alternate, sometimes opposite; living chaplets, of which each pearl is an individual. Each individual presents an oblong diaphanous or prismatic body, more or less symmetrical, and often furnished in front, rarely behind, with tentaculiform appendages. So great is the transparency, that the various organs may be observed through the skin as they perform their several functions.
Momus, an ancient philosopher, thought it a subject of regret that Nature had not thought of piercing the body with an opening sufficiently large for each one to see what was passing in the interior. The creature which now occupies our attention would surely have satisfied the demands of our critic: its body is, metaphorically speaking, a house of glass.
Fig. 126. Salpa maxima magnified (Forsk).