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



polymorplia. Two of these, Salpa rhomhoides aud Salpa polyniorpha, are probably 

 not Tunicata at all. Salp>a tricuspidata is probably the solitary form of Salpa 

 democratica-mucronata, and Salpa gihbosa the solitary form of Salpa costata-tilesii. 

 In the official account of the voyage ^ they figure various varieties of Salpa costata- 

 tilesii and two other species, Salpa lonyicaudata (probably the solitary form of Salpa 

 democratica-mucronata) and Salpa hirostrata (probably the aggregated form of 

 Salpa runcinata-fusiformis). A few years afterwards the same authors ' added to the 

 list the species Salpa hicaudata, Scdpa cordiformis, and Sallia microstoma. 



Chamisso^ had first discovered in 1819 the remarkable alternation of generations 

 which takes place in the life-history of Salpa, each species occurring in two forms, — the 

 solitary asexual and the aggregated sexual, — which are usually very unlike one another, 

 and are produced alternately. Kuhl and van Hasselt* afterwards discovered the 

 •well-known periodic reversal of the heart-beat ; and the circulation was more fully 

 investigated later by Eschscholtz ^ and by Milne-Edwards." Meyen" (1832) described 

 the nervous system and some other parts in the anatomy of Salpa. 



Eschricht* in 1841 gave a very full account of the species Saljxt cordiformis-zonaria, 

 with a description of the method of formation of the " chain," and excellent figures. 

 M. Sars ^ also gave an account of the process of gemmation in another northern form. 



The first good account of the reproductive organs is due to Krohn,'" who, in 1846, 

 wrote on the life-history and classification of the genus. This was followed in 1851 by 

 Huxley's memoir '^ on Salpa and Pyrosoma, which gave an account of his observations 

 made during the voyage of the " Eattlesnake," and independently of those of Krohn. 

 These two important works added considerably to the knowledge of both the structure 

 and the life-history of Salpa, and Krohn also did good service in clearing up the 

 synonymy of the species to a considerable extent, and in placing the aggregated and 

 solitary forms of the same species together. 



C. Vogt in 1854^' carried on the work begun by Krohn, and gave a very full account 

 of the embryology and life-history of Cyclosalp>a pinnata, with a shorter description 

 of other species. H. Mliller " and Leuckart " also about the same time contributed to 

 the knowledge of the structure and relations of Salpa, giving a good account of the 

 nervous system and sense organs, and of some parts of the development. 



Costa, ]\Iacdonald, JM'Intosh, and other zoologists have added more or less important 



' Freycinet, Voyage autour du Monde, Zool, Paris, 1824. 



- Ann. d. Set. Nat., torn. x. p. 225, 1827. ^ De animalibus quibusdam, etc., fasc. i., de Salpa. 



* Ann. d. Sci Nat., torn, iii., 1824. '^ Oken, I.sis, 1824. 



« Ann. d. Sci. Nat. (Zool.), siir. 2, torn, xiii., 1840. ' Acad. Cm. Lcop., Nova Ada, torn, xvi., 1832. 



* Anat. physiol. Unders^gelser over Salperne, A". Dan. Vidensk. Selsk., Afh. viii. p. 297, 1841. 



3 Fauna littoralis Norvegise, 1846. i" Ann. d. Sci. Nat. (Zool.), ser. 3, torn. vi. p. 110, 1846. 



'1 Phil. Trans., 18.51, part ii. p. 567. ^- Mem. de VList. Genev., torn, ii., 1854. 



1^ Verh. phi/s.-med. Gesdlsch. Wiirzlntrri, Bd. iii. p. 57, 1852 ; and Zeilschr. f. iciss. Ziml., Bd. iv. p. 329, 1853. 

 1* Zoologische Untersuchungen, ii., Salpa uud Verwandte, Giessen, 1854. 



