TUNICATA (Lat. nom. p1. of lunicalus, p. p. of tunicare, to clothe with a tunic, from tunica, tunic). The single class of the Uroehorda containing small marine animals inclosed in a soft elastic tunic, which opens by two aper tures (oral and anal), and contains tunicin, a substance resembling cellulose. The tunic is usu ally thick, tough, and leathery, but may be very thin and delicate, especially in some of the smaller forms. The mouth, supplied with tactile organs, opens into a capacious pharynx, which contracts abruptly into a narrow oesophagus, then expands into a stomach, followed by a more or less coiled intestine that terminates close to the atrial pore or orifice (the other opening in the tunic). The walls of the pharynx are per forated by numerous slits, through which the water entering at the mouth passes out into the cavity of the tunic and thence escapes through the atrial pore, carrying with it the waste mat ter, reproductive products, etc. The pharynx is thus an important respiratory organ and is corre spondingly richly supplied with blood vessels. The heart is peculiar, in that it reverses its beating, pumping the blood in one direction for a time, then pausing and presently pumping the blood in the other direction for an equal length of time. The nervous system is greatly reduced, and sense organs, except those of touch, are gen erally wanting. The muscular system is well developed in some forms, especially in the tunic, but in others it is reduced and serves chiefly for closing the oral and atrial pores. Reproduction is not only by eggs, hut takes place in many forms also by budding. In size and color the tunicates offer the greatest possible diversity. The smallest forms are, as individuals, only a few millimeters long, though the colonies which they compose are often many inches in diameter. The largest individuals are six or eight inches high, though some forms are on stalks of much greater length. In color we find some species jet black and some pure white, and others fire red, but dull shades of greenish, grayish, or brownish are more com mon. Many of the free-swimming forms are al
most transparent and nearly colorless.
The number of species of tunicates is large and the classification has varied greatly with different writers. They are a comparatively well defined group and are now considered as a considerably degenerated offshoot of the branch from which the Chordata have arisen. (See ASCIDIAN. ) The tunieates may be conveniently divided into three orders, the Larvalia, the Thaliacea, and the Ascidiacea. The La rvalia are a very small group, containing only one family and few species, remarkable for their small size and free-swimming habits, and the retention throughout life of certain characters which in the other orders occur only temporarily during their development. These larval characters are especial ly the tadpole form with a tail, the nerve cord, and the persistent notochord or urochord. The Thaliacea are also free-swimming, but are much larger and more degenerate or specialized forms. There are three families, one of which contains the beautiful sal pas The order Ascidiacea contains three suborders and a dozen or more families. One suborder is free-swimming like the salpas, and so is called the Salpiformes; the second, called Compositce, contains fixed forms which multiply extensively by budding and thus form compound organisms, the in dividuals of which are usually very small; the third suborder, the Simplices, are almost al ways fixed individuals (see ASCIDIAN ) ; but in many cases large numbers of individuals are united by a common stolon from which they have arisen by and these forms are sometimes known as social aseidians. The Shn plices are better known, popularly, than the other suborders, and many of them have popular names. as sea-pork, sea-peach, sea-potato, and sea-squirt.
Consult Herdman, "Report on the Tunicata," in Challenger Reports, vol. vi. (London. 1888).