6 THE VOYAGE OF H.M.S. CHALLENGER. 



The great Freneli circumnavigating expeditions of the third decade of the century, 

 although they made known the external appearances of some new forms amongst 

 pelagic Tunieates, contributed little to the anatomy, or any other real knowledge of 

 the group, and added not a little to the confusion iu which the synonymy was even 

 then beginning to be involved. 



In 1831, OiltOpleura, the best known genus of the Larvacea (Appendiculariidas), 

 was described by H. von Mertens, This observer had found this most important 

 pelagic form near Bering Strait in 1828, where a form, possibly the same, had been 

 seen and imperfectly characterised (under the name Appendicularia) by Chamisso 

 some years before. Mertens was fortunate enough to see it in the act of forming and 

 getting rid of its periodically produced temporary test, which has since borne the 

 German name " Haus," applied to it by Mertens. 



Anchinia, another important pelagic Tunicate, and its close ally Doliolum, were 

 made known in 1833 and 1835, the former being established by Eschscholtz, and the 

 latter by Quoy and Gaimard as one of the results of the voyage of the " Astrolabe." 



Costa's observations on the Mediterranean Salpm, those of Milne-Edwards on 

 Pyrosoma and on Salpa, and of Eschxicht on two little-known forms of Salpa [Salpa 

 coixliformis-zonaria), made in 1839-41, added considerably to the knowledge of the 

 anatomy and phj^siology of these genera. M. Sars and Krohn, during the decade that 

 followed, contributed important observations on the Salpidte — the latter author treating 

 more especially of the reproduction, the develoj)ment, and the life-history. Krohn 

 likewise did good service at this period by attempting to clear up the synonymy of 

 Salpa, and to range the two kinds of generations (the solitary and the chain forms) 

 under their proper species. 



By far the most important contributions, however, of the middle of the century 

 were Huxley's weU-known memoirs on the pelagic Tunicata Salpa, Doliolum, Pyrosoma, 

 and Ap>pendic'ularia. Huxley's observations upon these forms were made during the 

 voyage of H.M.S. " Eattlesnake," and, so far as Salpa is concerned, covered partly the 

 same ground as the independent observations of Krohn, published previously. The 

 memoir on Salpa treats mainly of the structure of two different forms of the genus 

 (evidently the " solitary" and "chain" generations oi Salpa dcmocratica-mucronata), 

 of their methods of reproduction, and of their relation to one another. He discovered 

 the testis in the chain Salpa, thoroughl}^ established the truth of Chamisso's descrip- 

 tion of the "alternation of generations," and pointed out the essential difference 

 between the two methods of reproduction occurring in the life -history. In his 

 observations upon Pyrosoma, which were made upon the living animal, Huxley 

 corrected and added to the descriptions of his predecessors, especially in regard to 

 the reproductive organs. He also in an important section of his memoir discussed the 

 relationships between the various groups of Tunicata, and pointed out the similarity 



