Page:Natural History, Mollusca.djvu/331

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319
SWIMMING ASCIDIANS.

manner, which would seem to make them composite animals, though they are not such; and he divides it into two tribes:—1. The simple Salpaceans (Salpa); 2. The aggregate Salpaceans (Pyrosoma).

"A great interest," observe Messrs. Forbes and Hanley, "is attached to the natural history of the Salpæ, on account of their singular mode of reproduction, discovered by the German naturalist Chamisso, and the extraordinary generalization to which that discovery in a great measure gave rise. Previous observers had noticed that these animals were sometimes found solitary, at others united together in long chains, composed of numerous individuals of similar form, each an independent being, though constantly associated, and linearly aggregated with its companions. These long chains swim through the tranquil water with regular serpentine movements; for the creatures of which they are composed contract and expand simultaneously, keeping time, as it were, like a regiment of soldiers upon parade. Each chain seems, consequently, to be a single being, acting through the influence of an unique will; and hence sailors often look upon it as a reptile; and in many seas the Salpa-chains are called sea-serpents. But when taken out of the water, the links of the chain fall asunder, the several distinct animals of which it is composed suddenly losing their power of adhesion. In consequence of accidents, broken-up chains and separated members of such communities are not unfrequently met with, in seas where Salpæ are numerous. But other Salpæ are also met with, very dissimilar in form, and never united together in chains. Now, the discovery of Chamisso was,