Page:Natural History, Mollusca.djvu/330

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Other genera in this family form similar encrusting masses, but the animals are placed in irregular tortuous lines instead of stars, and the two orifices are near together. Others do not encrust foreign substances, but are grouped in variously shaped knots, or fruit-like bodies, adhering to stones and shells.

Family Salpadæ.

(Swimming Ascidians.)

The body in this group is free, or not adherent; more or less cylindrical; with a thick external envelope, which is somewhat cartilaginous; transparent; having the two orifices, which are ordinarily very large and distant, nearly terminal, one at each extremity. The branchiæ, in the form of a narrow band, traverse obliquely the respiratory cavity of the receiving orifice to the aperture of the mouth.

M. de Blainville remarks, that one may easily perceive the relationship of this family to the other Tunicata, by supposing an Ascidia slit between the two tubes which terminate it, and then extended lengthwise. It is then, he observes, easy to determine the analogy of the apertures, of which neither the one nor the other is properly any more the mouth or the anus than in the Ascidiæ; but one (the widest, the greatest, and the most distant from the mouth), is the entrance of the incretory or respiratory tube, and the other is that of the excretory tube. He adds, that the species of this family are, like those of the preceding, susceptible of living solitary, or aggregated in a fixed