“This light I do not think could have been produced without the agency of electro-magnetic currents exercising their exciting influence upon some organic animal or vegetable substance. And one thing I wish to point out is, that, whilst the ship was stopped and the light yet some distance away, nothing was discernible in the water; but, so soon as the light reached the ship, a number of luminous patches presented themselves: and, as these were equally as motionless as the ship at the time, it is only natural to assume that they existed, and were actually in our vicinity, before the light reached us, only they were not made visible till they became the transmitting media for the electro-magnetic currents. This hypothesis is borne out by the fact that each wave of light in its passage was distinctly seen to pass over them in succession; and, as the light gradually became less brilliant, they also became less distinct, and had actually disappeared so soon as the waves of light ceased to exist.” A little Ascidian called the _Salpa_ is quite famous for its luminous properties. Like the previously mentioned form, it is a free swimmer, two kinds of individuals being recognized. One is known as solitary; while the others are termed chain zoöids, being many joined together, forming long chains, the links represented by the individual animals. The _Salpa spinosa_, a familiar form upon our coasts, is quite cylindrical, often a little flattened above and below, and seemingly moulded in glass, so beautiful is its structure upon examination. As small and common as they are, they have created much discussion. Some observers deem their development one of the most remarkable instances of the alternation of generations. Chamisso, the German poet-naturalist, explains the relationship as follows: “A _Salpa_ mother is not like its daughter or its own mother, but resembles its sister, its granddaughter, and its grandmother.” Dr. W. K. Brooks has given much attention to these forms in this country; and, from his point of view, the alternation of generations would be impossible. The _Salpæ_ give little signs of animation. “The only conspicuous vital action,” says Professor Owen, “is the rhythmical contraction and expansion of the mantle, in which the elasticity of the outer tunic antagonizes the contraction of the inner one. During expansion, the sea-water enters by the posterior aperture, and is expelled, in contraction, by the anterior one; its exit by the opposite end being prevented by a valve. The re-action of the jet, which is commonly forced out of a contracted tube, occasions a retrograde movement of the animal.” As they move along, on dark nights, they present the appearance of fiery serpents or luminous ribbons (Plate XVI., Fig. 1), winding their way over the sea,--a most striking spectacle. [Illustration: PLATE XVI. CHAIN OF SALPS _Salpa spinosa._ _Appendicularia._]