44 PHYLUM TUNICATA (UROCHORDA). 



join round the pharyngeal process (Fig. 37, eb). The test of the primary 

 tetrazooid colony is said to be formed entirely by the cyathozooid, but in 

 the subsequent budding each new zooid plays its part in adding to it. 



The stolon undergoes fission by transverse constrictions into buds. 

 First one bud is marked off (Fig. 36 A), thenanother between the first and 

 the parent (Fig. 36 C), and so on until five have been formed, the youngest 

 bud being always next the parent. When five zooids have thus been 

 marked off, the distal one has acquired full development and becomes de- 

 tached and a new constriction is formed at the base of the stolon. The 

 new zooids when detached pass round and take up their position near the 

 common cloacal opening of the colony, where the youngest zooids lie. As 

 they increase in age they become further and further removed from this 

 opening by the interposition of the continually forming new zooids. 



The points in the above account which must be received with caution 

 are those relating to the origin of the nervous system and peribranchial 

 tubes. In the stolon of the cyathozooid these organs arise as ectoderm 

 invaginations ; in the stolons of the later zooids which are directly derived 

 from that of the cyathozooid, they are said to arise from mesoderm. It 

 is difficult to believe that this difference really exists. 



Order 2. THALIACEA.* 



Free-swimminy solitary pelagic forms, which in the adult are 

 never provided with a tail or a notochord. Metagenesis always 

 occurs, and the sexual forms typically remain for some time con- 

 nected to a process of the body of the asexual form by which they are 

 budded. The mouth and atrial apertures are at opposite ends of the 

 body. 



The test is permanent and transparent, and closely adherent 

 to the body. The musculature of the mantle is in the form of 



* Traustedt, Spolia atlantica, Bidrag til Kundskab om Salperne, Danske 

 Vid. Selsk. Skrift., 1885, p. 339. Kowalevsky and Barrois, Materiaux 

 pour servir a 1'histoire de VAnchinia, Journ. de VAnat. Phys., 19, 1883. 

 Uljanin, Die Arten der Gattung Doliolum im Golfe v. Neapel, Leipzig, 1884. 

 Barrois, Sur le cyclique genetique et la bourgeonnement de 1'Anchinie, 

 Journ. de VAnat. Phys., 21, 1885, p. 193. Seeliger, Die Knospung der 

 Salpen, Jen. Zeitsch., 19, 1886. Korotneff, Die Knospung der Anchinia, 

 Z.f.w.Z., 40, 1884, p. 50. Id., La Dolchinia mirabilis, Naples Mit., 10, 

 1891, p. 187. Id., Embryologie der Salpa democratica, Z.f.w.Z., 59, p. 29, 

 1895. Id., Tunicatenstudien, etc., Naples Mit., 11 and 12, 1895-6. Id., 

 Zur Emb. v. Salpa runcinata-fusiformis, Z.f.w.Z., 62, 1896. Brooks, The 

 Genus Salpa, Mem. Biol. Lab. Johns Hopkins University, 2, 1893. Salensky, 

 A series of papers on the development and budding of Salpa, vide, Z.f.w.Z., 

 17, 1876, 30, 1878, Morph. Jahrb., 3, 1877, 20, 1893, Naples. Mit., 4, 1883, 

 11, 1895. Heider, Beitrage zur Embryologie von Salpa fusiformis, Abh. 

 Senck. Ges. Frankfurt, 18, 1895, p. 367. Metcalf, Follicle cells of Salpa, 

 Zool. Anz., 20, 1897, and on the eye and subneural gland in Brooks' 

 Monograph, loc. cit. Id., Notes on the Morphology of the Tunicata, Zool. 

 Jahrb., Anat., 13, 1900, p. 495. 



