Degenerate Chordata, which are either simple or compound (colonial), fixed or free, with a test usually containing cellulose, and secreted by the ectoderm. The nervous system is reduced to a single ganglion, except in one group (Larvacea); the notochord confined to the tail, which, with the exception of the same group, is a provisional or larval structure. There is a single or double exhalent aperture by which the water escapes that has traversed the pharynx Hermaphrodite. There is usually a metamorphosis.
The class contains three distinct orders. The Larvacea are small in size, and have a flattened swimming tail moveably attached near the posterior extremity of the body on its ventral surface, towards which it bends. The Thaliacea are free-swimming, and more or less barrel-shaped. Among the Ascidiacea, the Ascidiae Salpaeformes (Pyrosoma) are colonial, and the colony has the form of a cylinder closed at one end, open at the other, with the Ascidiozooids placed vertically to the surface, in a common test or Ascidiarium; the A. Compositae form fixed colonies with the Ascidiozooids contained within an Ascidiarium, sometimes grouped into systems or coenobii; and the A. Simplices are fixed, and either solitary and of a compressed shape, or united by creeping stolons (Clavelinidae).
The ectoderm consists of a single layer of polygonal cells. The cells of the anterior region of the body in Larvacea are large, and secrete a hyaline gelatinous and sticky substance, which forms the so-called 'house.' This house is sometimes of great size, and extends over the swimming tail. It is separated by one cavity from the greater part of the body, and by another from the tail, which moves freely within. It is detached and renewed at short intervals. When the house is small, the ectoderm cells left uncovered by it are modified, and in Fritillaria urticans some of them develope thread cells. The test is very thin and delicate, and is shed from time to time in Doliolum. It is thicker in other Urochorda, and either transparent or when very thick more or less opaque, gelatinous or tough, and in the Ascidiae Compositae and Salpaeformes it constitutes a continuous investing mass in which the zooids are lodged. The substance of the test is chiefly cellulose except in Doliolum; it is often fibrillated, and in some A. Compositae calcareous deposits occur in it, especially in autumn. The cells of the ectoderm generally proliferate and wander into it, and the cells thus imbedded may become pigmented or vacuolate.
It is also often penetrated by blood-vessels. The body walls are composed of muscular and connective tissues. The muscle fibres are fusiform or filiform, and not striated. The fibres form sphincters round the oral and the atrial apertures. Elsewhere they may be arranged either irregularly, or in longitudinal and circular layers, or in circular hoops round the barrel-shaped body (Thaliacea) which are either independent (Cyclomyarid) or united (Desmomyaria). In Larvacea the muscles are striated (Fol), and are restricted to the tail, where they form two bands, which are segmented into myomeres. The connective tissue cells are often pigmented. The caudal notochord of the Larvacea and of the larva is rod-like, and composed of a clear substance, like cartilage in consistency, inclosed within a delicate sheath, beneath which are remains of the cells of which the tail was originally composed. In the larva the clear substance appears first of all as a series of discs one behind the other, which ultimately fuse.
The nervous system in the larva consists of two anterior dilatations, the foremost vesicular, the second with a fine canal, and of a posterior nerve, which runs along the tail. At the root of the tail there is a ganglionic enlargement, from which a pair of nerves passes off to the corresponding myomere. Similar pairs of nerves are given off to succeeding myomeres. Their points of origin are dilated, but apparently not ganglionic in all instances. The anterior dilatation of the brain opens externally by a pore, left when the neural groove closes to form the neural tube. In Larvacea the nervous system is somewhat similar. There is an anterior pyriform ganglion, connected by a nerve containing an internal fine canal to a ganglion at the root of the tail. The caudal nerve gives off successive sets of nerves, and lies on the left side of the tail, owing to a twist in that organ. In other Urochorda the nervous system is either from the first a simple ganglion, or becomes reduced to it during the metamorphosis. This ganglion gives off branching nerves, both anteriorly and posteriorly1. A process of the ganglion in Larvacea and Thaliacea extends towards a ciliated tubular depression opening dorsally and anteriorly into the pharynx, in front of the peripharyngeal bands.
The cavity of the ganglion has been said to open into this tube in the larva and the young Salpa. The ciliated depression (= ciliated sac, dorsal or olfactory tubercle) alluded to is found universally in Urochorda; and in many Ascidiae Sim-plices and A. Compositae a system of glandular tubes, surrounded by blood sinuses, underlies the nerve ganglion, and opens into the depression. This neural gland has been considered to be renal in function, and to be the homologue of the pituitary body of Vertebrata. As to organs of special sense, the oral tentacles of Ascidiidae, etc, are probably tactile in function. So too certain modified ectoderm cells bearing cilia, surrounding the oral aperture of Larvacea, and fringing even the margins and tip of the tail in Kowalewskaia. They occur also on the lobes of the oral and atrial apertures in Doliolum and Salpa, as well as on the body surface. Orange-red spots, probably visual in function, are found, one between each of the lobes surrounding the oral aperture in many Ascidiidae. Pyrosoma and Salpa have a pigmented outgrowth of the ganglion, with a refracting body imbedded. And the larva has an eye with a concave pigmented retina and complex lens within the anterior dilatation of the nervous system.