In order to move itself, the Salpa has recourse to a singular artifice. It introduces water into its body through a posterior opening, furnished with a valve, which it expels by an anterior outlet situated near the mouth. It is thus pushed backwards, and swims, as it were, by recoil. Moreover, it swims with its belly upwards. All the elements of a chain of Salpas act in concert; they contract and dilate simultaneously; they advance as a single individual. One of them floats on the surface with the undulations of a serpent, so that among sailors they have gained the appellation of sea-serpents. These long, living trains abound in the Mediterranean, principally towards the African coast, and in the Equatorial seas. They are inhabitants of the open sea, and live immerged at considerable depths; but when the nights are calm they show themselves on the surface. As they spread themselves abroad with a strong phosphorescent light, they resemble long ribbons of fire, unrolling their long waving lines in spite of the waves, as in Fig. 127. What wonders they see who go down into the great deep! What sights are reserved for the navigator who traverses the Tropical seas during the silence of night!
When a chain of Salpas is drawn from the water, the rings separate, and they can no longer be made to adhere. The social bond has been dissolved by a superior force.
Salpas are sometimes met with, isolated and solitary, whose exterior conformation differs much from that which is proper to the connected Salpa; so different, indeed, that it might belong to another type. Chamisso, Krohn, and Milne Edwards have ascertained that the Biphora is viviparous, and that each species is propogated by alternate generation, the young creature being unlike its immediate parent. One of these generations is represented by the solitary individuals, the other by an aggregation of individuals. Each solitary Biphora engenders a new group—a chain; each constituted member of the chain engenders a solitary Salpa.
Fig. 127. Phosphorescent chain of Salpas on the surface of the sea.
Thus a Salpa is not organized like its mother or daughter, but rather like its sister, its grandmother, or granddaughter—another example of alternate generation, which has already been discussed in treating of zoophytes.
Those marine creatures which pass their lives in a forced community—animals which eat, sleep, or rest always in company—who abandon themselves together to the soft caresses of the waves,—these colonies, or, rather, republics of animals, leading constantly the same monotonous existence,—reveal to us very strange things: an identical community of sentiments in a crowd of beings riveted by the same chain—a chain at once physical, intellectual, and moral!