ASCIDIAN, a marine animal, so called from Ascidia, a genus of Tunicata. Ascidians were once regarded as mollusks, and afterward as worms, but when their embryology and early stages were studied and it was found that they passed through a tadpole-like stage, in which the tail is supported by a notochord, and that in other respects they approached the verte brates, they were placed with the vertebrates in the group Chordata. The simple ascidians at tain to a large size, A. callosa being about two inches in diameter, quite round, and in shape and color much like a potato. The "sea-peach" (Cynthia pyriformis) is of the size and general shape of a peach, with its rich bloom and red dish tints. It is common at a depth of 10 to 50 fathoms on both sides of the north Atlantic. While other forms, as Boltenia, are stalked and fixed to the bottom, certain pelagic forms, as Pyrosoma and Salpa (q.v.), are free-swimming.
The compound ascidians, such as Amarcecium, grow in white or reddish masses on sea-weeds, rocks, shells, etc., the individual animals being minute. The interesting form Perophora grows in bunches on piles and wharves on the south ern coast of New England; it is perfectly trans parent, so that the heart and circulation of the blood can readily be observed under the micro scope. The heart is a straight tube, open at each end; after beating for a number of times, throwing the blood with its corpuscles in one direction, the beatings or contractions are regu larly reversed, and the blood forced in an oppo site direction. For a general account of the anatomy, development and metamorphoses of these animals, see TUNICATA.