Both dorsal and ventral cardiac vessels give off branches to the test. The blood has a clear plasma with nucleated and usually rounded corpuscles, many of which are sometimes pigmented (opaque white, yellow, red, brown, purple, blue). Specialised renal glands are not present (? the neural gland). Many Ascidiidae, however, have clear vesicles in masses round the intestine and in the body walls, containing concretions. And in the Molgulidae there is a sac-like organ close to the pericardium, containing rounded concretions, in which uric acid has been detected. A yellowish-green mass usually coats the first part of the intestine in A. Compositae, and perhaps corresponds to the clear vesicles of the Ascidiidae.
All Urochorda are hermaphrodite, but the male and female organs appear to become mature at different times. Sexual organs are absent, or at least atrophied, in the nurse forms of Salpa and Doliolum, in the Cyatho-zooid of Pyrosoma, and in certain generations of Botryllus. Both testes and ovary in Larvacea are simple saccular organs, devoid of ducts. They lie behind the stomach, and their contents are set free by dehiscence of the body walls, and the organism then dies. In other Urochorda the organs are either more or less saccular, as in Ascidia, or else branching tubes, and their ducts either open close together or by a common aperture, as in Doliolum, into the atrial cavity, and close to the anus. There is but a single ovum in Pyrosoma and Salpa. In most Urochorda except the Larvacea, follicular cells surround the ovum, and are inclosed with it in a membrane. The follicle cells sometimes give origin to a chorion enveloping the ovum, and in some instances grow out into long external villiform processes.
Within the chorion appear in many instances a number of so-called test-cells. The origin and fate of both the follicular and the test-cells are involved in much difficulty (see lit. cited below). Impregnation and sometimes development take place in the atrial cavity, in a special incubatory pouch opening near the anus (some A. Compositae), or within the ovary, as in Pyrosoma and Salpa. Segmentation is regular, except in Pyrosoma, where it is meroblastic, a germinal disc being formed. There is an invaginate gastrula, or in Molgula (? all species) a gastrula by overgrowth. In Salpa the developing embryo is nourished by a placenta formed, in part at least, by follicle cells; and certain cells - gonoblasts - derived from the follicle cells have been stated to take the chief part in the formation of the embryo itself (Salensky), but details vary in different species. Except in Salpa, Pyrosoma, and Molgula tubulosa, there is a larva which resembles in many respects one of the Larvacea. It possesses a straight swimming tail, supported by a notochordal rod, moved by lateral muscles, and containing an extension of the nervous system.
After a certain period of free existence, the larva attaches itself by means of the glutinous secretion of glands borne by three papillae developed at the anterior end of the body. The papillae subsequently atrophy, and the organism remains attached by processes of the test. The tail atrophies, and some of its cells appear to become blood-corpuscles, whilst the shape of the body undergoes profound changes. Except in the Larvacea, in the sexual forms of Thaliacea, and the majority of A. Simplices, asexual generation takes place by budding or fission. In the Clavellinidae there is a creeping stolon which produces buds, and the new organisms remain in vascular connection with the parent. In the A. Compositae and A. Salpae-formes colonies are formed in which the individuals are enveloped in a common test. As to the Compositae the larva buds in Distaplia and continues to develope, whereas in Pseudodidemnium gelatinosum it dies away, and in Amareuciumproliferum the post-abdomen (i.e. hind section of the body) divides transversely into several pieces, each of which becomes a new zooid, whilst the parent grows a new post-abdomen. In Botrylitis the first individual is asexual, buds, dies away, and this process is continued for several generations.
The zooids thus formed are arranged in star-like groups (systems or coenobii) round a common cavity, into which their atrial pores open. The germinal disc in Pyrosoma developes in the posterior region into a transitory Cyathozooid, in the anterior into the four first Ascidiozooids of the colony, which are connected to or continuous with the Cyathozooid. Alternation of generations occurs in the Thaliacea. The asexual generation or nurse has a ventral posterior stolon. This stolon grows to a great length in Salpa, and is constricted into a series of buds, which remain connected for a lengthened period, and are set free collectively from the parent as sexual or chain Salpae. The chain is eventually broken up. In Doliolum the stolon is small. It developes a number of primitive buds, which are set free, and creep over the parent by means of pseudopodial processes of the ectoderm cells. They become attached to a dorsal and posterior process of the body, where they multiply by transverse division, and are arranged in linear series, two lateral and one median. The dorsal process lengthens as the number of buds increases. The lateral series of buds grow into nutritive zooids.
The epithelium of the stolon, and of the bases of these zooids, is peculiarly modified, and the products of digestion appear to pass from the zooids to the parent, which loses its pharynx and digestive tract whilst its muscle-bands enlarge. The zooids of the middle series grow, and are eventually set free as foster-mothers. The foster-mother is asexual, and carries away attached to its peduncle a few primitive buds, which divide, forming fourteen to twenty secondary buds. These develope into the sexual Doliolum. The ovum in both Salpa and Doliolum produces the nurse. Anchinia probably resembles Doliolum more or less closely. The cylindrical body with attached buds, which become sexual animals or produce creeping buds, is probably a detached part of a dorsal (?) process. The nurse is not known unless it is the form from Naples described by Wagner.
The Urochorda are divisible into the three following Orders:
Free-swimming; body small, more or less oval, provided with a long swimming tail, containing a notochordal rod; a temporary gelatinous case, secreted by the ectoderm, which is renewed from time to time; two stigmata: Appendicularidae (Oikopleura, Fritillaria, Kowalewskaid).
Either sessile, and then simple, social, or compound, or free-swimming and colonial. Test well developed, and often massive; stigmata numerous, and pharynx large and specialised.
Fixed or free, solitary or social: sometimes producing by gemmation colonies, in which the Ascidiozooids are connected by a common vascular system, but each retains its own test. The oral and inhalent apertures are near to one another, at the same end of the body. Molgulidae (free as a rule), Cynthiidae, Ascidiadae, and Clavellinidae (colonial).
Fixed; colonial; Ascidiozooids imbedded in a common test; often connected by a common vascular system; and generally grouped more or less regularly in systems round a central cavity, into which the exhalent apertures of the zooids open. Body simple (Botryllidae); divisible into a 'thorax' and 'abdomen' (Didemnidae); or into a 'thorax,' 'abdomen,' and 'post-abdomen' (Polyclinidae). Gemmation universal.
Free-swimming pelagic colony, in the form of a hollow cylinder, closed at one end and open at the other, at which its extension takes place. Zooids placed perpendicularly to the surface, with the oral aperture external, the cloacal internal, and leading into the central hollow of the cylinder. Pyrosomidae with one genus Pyrosoma, which is phosphorescent.
Free-swimming; more or less barrel-shaped, with oral and cloacal apertures at opposite ends of the body; test very thin; muscles circularly arranged; and viscera contracted into a small compass, and laterally placed. An alternation of generations. Doliolidae (= Cyclomyaria) the muscles in complete hoops; two transverse rows of stigmata; Doliolum; Anchinia. Salpidae (=Desmo-myarid) muscular hoops incomplete and sometimes uniting; pharynx reduced to the dorsal lamina, on either side of which is a large space; Salpa.
Larvacea, Fol, Etudes sur les Appendiculaires, Geneva, 1872. Vertebration of tail, Ray Lankester, Q. J. M. xxii. 1882. Ovogenesis, etc, in Appendicularia, A. B. Lee, Recueil Zool. Suisse i, 1884.
Ascidiacea, see pp. 106-7. Ascidians of Provence, Roule, An. Mus. Hist. Nat. Marseille, ii. 1884: Id. Recueil Zool. Suisse, iii. 1886; Id. A. Sc. N. (6) xx. 1886. On place of Clavellinidae, Herdman, Proc. Roy. Soc. Edin. x. p. 716. Development of Clavellina, Seeliger, J. Z. xviii. 1885. Ascidiae Compositae, Herdman, Challenger Reports, xiv. 1886, and Nature, xxix. 1883-84. Synascidiae, Giard, A. Z. Expt. i. 1872; ii. 1873. Botryllus, Krohn, A. N. 35, 1869.
Pyrosoma, Keferstein und Ehlers, Zool. Beitrage, Leipzig, 1861; Huxley, Tr. L. S. xxiii. 1862; Kowalewsky, A. M. A. xi. 1875.
Thaliacea. Doliolum, Uljanin, Fauna und Flora des Golfes von Neapel, x. 1884. Salpa; testis and alternations of generation, Salensky, Z. W. Z. xxx. Suppl. 1878; gemmation, Id. M. J. iii. 1877; Seeliger, J. Z. xix. 1886; development of ovum, Salensky, Mittheil. Zool. Stat. Naples, iv. 1883. Anchinia. Wagner, A. Z. Expt. (2) iii. 1885; Korotneff, Z. W. Z. xl. 1884; Kowalewsky and Barrois, A. N. H. (5) xii. 1883.
Egg and envelopes in Urochorda, Fol; Sabatier, Recueil Zool. Suisse, i. 1884; and Roule, ibid. ii. 1885. Salensky on Salpa (supra); Seeliger on Clavellina lepadi-formis, SB. Akad. Wien, lxxxv. Abth. i. 1882. Segmentation and postembryonal development, E. van Beneden et Julin, Archives de Biol. v. 1884; vi. 1886.
For literature, see Herdman, Tunicata, Challenger Reports, vi. 1882; xiv. 18861.