The Tunicaries are exclusively marine in their distribution, and are principally littoral and shallow-water forms, though some are found at considerable depths, and many are pelagic in habit. The singular Salptdae have the branchial and atrial apertures placed at opposite ends of the body, and are found swimming in the open sea, often in immense shoals. The Appendiculariae, with their permanent larval tails, are likewise oceanic, as is the cask-shaped Doliolum. Lastly, in Pyrosoma, we have a singular compound oceanic Tunicate, in which the numerous zooids form a tubular colony, which is propelled through the water by the united excurrent respiratory jets of its component members. Like the Salpians, it is brilliantly phosphorescent. - On the other hand, the more typical Tunicates are found attached to all kinds of submarine objects, or (as in Pelonaia) embedded in mud.
During the "Challenger expedition," some singular deep-sea Tunicates were obtained, and have been since described by Mr Moseley. One of these (Hypobythius) was found in the Pacific, at a depth of nearly 3000 fathoms, attached to foreign objects by a peduncle. Its test is hyaline and transparent, and is strengthened by symmetrically disposed cartilaginous plates. Octacnemus, dredged at over 1000 fathoms, is also hyaline- and transparent, with a short stalk, but it possesses eight long radiating processes, which give it a stellate appearance; and the branchial sac is so flattened as to become nearly horizontal.
No Tunicates are known with certainty to have been preserved in the fossil condition, but the singular Silurian genus Pasceolus has been doubtfully referred to this class.