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. [Illustration: 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! CHAPTER XI. ACEPHALOUS MOLLUSCA. "Sigiliatim mortales, cunctum perpetul." APULEIUS. The Mollusca proper were divided by Cuvier into five great classes:-- I. Lamellibranchiata, or Acephalous Mollusca, often called Conchifera. II. Brachiopoda. III. Gasteropoda. IV. Pteropoda. V. Cephalopoda. The name Mollusca indicates the characters which most struck the ancients: they are soft--in Latin, _mollis_: their flesh is cold, humid, and viscous. In consequence of their very softness, they are generally furnished with an apparatus of defence or protection, in the shape of a calcareous cuirass, called a _shell_. According to the species this test is a coat of mail, a buckler, or a tower. The mollusc is thus armed and defended against all attacks from without, nearly after the manner of a knight of the middle ages; only the knight was not quite shut up in his armour, while the mollusc is attached to it by indissoluble organic bonds. "Such a life and such a habitation!" says Michelet. "In no other creature is there the same identity between the inhabitant and the nest. Drawn from its own substance, the edifice is the continuation of its fleshy mantle. It follows its form and tints. The architect has communicated its own substance to the edifice."