STRUCTURE AND MODE OF LIFE.—This is one of the most interesting groups of the Tunicata, as it shows more completely than any of the rest the probable characters of the ancestral forms. It has undergone little or no degeneration, and consequently corresponds more nearly to the tailed, larval condition than to the adult forms of the other groups. It retains, in fact, the originally posterior, chordate, part of the body which is lost in the metamorphosis of all the other Tunicata. Hence the Appendicularians have been described as permanent, or sexually mature, larval forms, and hence also the adult _Ascidia_ may be said to correspond to the trunk alone of the Appendicularian. The Order includes a single group, the APPENDICULARIIDA, all the members of which are minute (usually about 5 mm. in total length) and free-swimming (Fig. 28). They occur near the surface of the sea (and exceptionally in deeper water) in most parts of the world, moving in a characteristic vibratory manner by the contractions of the powerful tail (see Fig. 27). They possess the power of forming with great rapidity, from tracts of specially large glandular ectoderm cells, the "oikoplasts," an enormously large (many times the size of the body) investing gelatinous layer, which probably corresponds to the test of other groups, although it is doubtful whether it contains cellulose, and it differs also in having no immigrated cells and in its temporary nature. This structure (Fig. 28) was first described by Von {66}Mertens, and by him named "Haus"; it has recently been more minutely investigated by Lohmann. It is only loosely attached to the body, and is frequently thrown off soon after its formation. Its function is probably protective, and possibly to some extent hydrostatic, and it may also be of use in straining the nutritive particles from the large volumes of water which filter through its complicated passages and perforated folds.[98] The long, laterally compressed "tail" in the Appendiculariida is attached to the ventral surface of the body (Fig. 30), and is bent downwards and forwards, so that it usually points more or less anteriorly; and is twisted through an angle of 90°, so that the dorsal edge lies to the left. It shows what have been interpreted as traces of metameric segmentation, having its lateral muscle-bands broken up into successive pieces (supposed myotomes, probably only cells), while the nerve-cord presents a series of enlargements formed of groups of nerve-cells from which distributory nerves are given off. In _Oikopleura_ the muscle-band in the tail is formed of ten cells fused on each side. Near the base of the tail there is a distinctly larger elongated ganglion. The urochord in the tail consists of a homogeneous rod surrounded by a sheath containing nuclei.