The species of this family are very numerous: nearly fifty are enumerated as British; and probably not half of the whole number have yet been described. One of the most common is Ascidia virginea, which grows to an inch and a half in length, and about three quarters in breadth. It is pellucid and crystalline, of a pale yellowish tinge, revealing through the test the branchial sac, spotted with crimson, and crossed with lines of white. It adheres to stones, dead shells, and living seaweeds in deep water.
In essential points these resemble the preceding family, but the individuals are not distinct, but united by a common root-thread, from which they spring, like buds from a creeping root-stock. The thread creeps over the surfaces of stones, the stems of sea-plants, &c., continually growing by a lengthening of its extremity, and increasing by throwing out, either in groups or at regular intervals, a kind of buds, that develop into Ascidian mollusks, which commonly stand on more or less distinct foot-stalks.
The family may be illustrated by the accompanying figure, greatly magnified, of a tiny species (Perophora Listeri), found occasionally on our own coasts. I obtained the individual from which the figure was taken at Ilfracombe, attached to a fragment of sea-weed. It is, to the naked eye, a globule of clear jelly, not larger than a pin's head, yet disclosing, under the microscope, an elaborate