91 8 POLYZOA AND TUNICATA 



one on either side of the entrance to the stomach, are prolonged from 

 it to the external surface ; and by the action of the long cilia with 

 which these are furnished, in conjunction with the cilia of the 

 branchial sac, a current of water is maintained through its cavity. 

 From the observations of Huxley, however, it appears that the 

 direction of this current is by no means constant ; since, although it 

 usually enters by the mouth and passes out by the ciliated canals, 

 it sometimes enters by the latter and passes out by the former. The 

 caudal appendage has a central axis (notochord), above and below 

 which is a ribbon-like layer of muscular fibres ; a nervous cord, 

 sfcudded at intervals with minute ganglia, may be traced along its 

 whole length. By Mertens, one of the early observers of this animal, 

 it was said to be furnished with a peculiar gelatinous envelope or 

 Ifaus (house), very easily detached from the body, and capable of 

 being re-formed after having been lost. Notwithstanding the great 

 numbers of specimens which have been studied by Muller, Huxley, 

 Leuckart, and Gegenbaur, none of these excellent observers has 

 met with this appendage; but it Jias been since seen by Allman, 

 who describes it as an egg-shaped gelatinous mass, in which 

 the body is imbedded, the tail alone being free ; whilst from either 

 side of the central plane there radiates a kind of double fan, which 

 seems to be formed by a semicircular membranous lamina folded 

 upon itself. It was surmised by Allman, with much probability, that 

 this curious appendage is ' nidamental,' having reference to the 

 development and protection of the young ; but on this point further 

 observations are much needed ; and any microscopist who may meet 

 with Appendicularia furnished with its * house ' should do all he can 

 to determine its structure and its relations to the body of the 

 animal. 1 



1 For details in respect to the structure of Appendicularia, see Huxley in Phil. 

 Trans, for 1851, and in Quart. Journ. of Microsc. Sci. vol. iv. 1856, p. 181 ; also 

 Allman in the same journal, vol. vii. 1859, p. 86 ; Gegenbaur in Sieboldund Kolliker's 

 Zeitschrift, Bd. vi. 1855, p. 406 ; Leuckart's Zoologische Untersuchungen, Heft ii. 

 1854 ; Fol's ' Etudes sur les Appendiculaires ' in Archiv. Zool. exper. torn. i. 1872, 

 p. 57'; the three memoirs by H. Lohmann published in 1896. For the Tunicata 

 generally, see Professor T. Rupert Jones in vol. iv. of the Cyclop, of Anatomy 

 and Physiology; Professor Herdman's article, 'Tunicata,' in the 9th edition 

 of the Encyclopedia Britannica ; Mr. Alder's ' Observations on the British 

 Tunicata ' in Ann. of Nat. Hist. ser. iv. vol. xi. 1863, p. 153 ; and Mr. Hancock's 

 memoir ' On the Anatomy and Physiology of the Tunicata ' in the Journal of the 

 Linnean Society, vol. ix. p. 309. Great additions to our knowledge have been 

 made by Professor Herdman, whose reports on the forms collected by H.M.S. 

 Challenger should be consulted, and by Professors Van Beneden and Julin (see espe- 

 cially their memoirs in the Archives de Biologie) . See also Eoule, ' Recherches sur les 

 Ascidies simples des cotes de Provence,' Ann. Museum Marseilles, ii. ; Seeliger, 

 ' Die Entwickelungsgeschichte der Socialen Ascidien,' Jenaische Zeitschr. xviii. 

 p. 528 ; Salensky, ' Neue Untersuchungen iiber die embryonale Entwickelung der 

 Salpen,' Mitth. Zool. Stat. Neapel, iv. pp. 90, 327 ; and Ulianin, ' Die Arten des 

 Gattun'g Doliolum im Golfe von Neapel,' in the Fauna und Flora des Golfes von 

 Neapel, x. The above titles by no means exhaust the list of recent important memoirs 

 on Tunicata, but the researches of Caullery, Metcalf, Pizon, and Seeliger are beyond 

 the scope of this work. The last-named has commenced a systematic account of the 

 group in Bronn's ThierreicJi. 



