TOO THE VOYAGE OF H.M.S. CHALLENGER. • 



that summer in Torquay liarlujur. They were evidently species of the two genera 

 Oikopleura and FritiUaria. 



The only papers of any importance which have appeared since that date are one by 

 Eisen ^ on a new species, described under the name of Vexillaria speciom ; one by J. 

 Barrois " on a new species from the English Channel ; two by Langerhans," including 

 a list of the Appendieulariaus of Madeira, with a description of two new species of 

 Oikopleura ; and a short paper by Eay Lankester'' on the vertebration of the tail, 

 which he had observed in some species of the group. 



The characters of this well-defined group are very important and interesting. The 

 body-form, with the well-developed tail, is the most distinctive feature (see Figs. 

 12 and 1.3). The tail is attached, not at the posterior end of the body, but on the 

 ventral surfoce near the middle, and in a state of rest it generally inclines forwards. 



The tail agrees in essential structure with that of the larval Ascidian. It is 

 traversed by a notochord, or urochord, placed between plates of muscular tissue divided 

 into sections or myotomes." On the left side of the urochord lies a nerve cord, 

 corresponding to the neural canal of the larval Aibcidian, and having slight ganglionic 

 enlargements at intervals. From this cord motor nerves are given ofi' to the groups 

 of muscle fibres ; the tail thus shows distinct traces of metameric segmentation. The 

 outside of the tail, like the rest of the body, is formed of the ectoderm, which produces 

 on occasions the thickened and curiously-shaped "Haus," the homologue of the test of 

 other Tunicata (see Fig. 12). 



Fig. 12.— Oikopleura in "Haus," from the Encycloiifficlia Eiitannica, 9th ed. (after Fol). 



This structure may be many times the size of the body proper, and has quite a 

 different shape. It is only loosely attached to the ectoderm, and there are passages in 



1 K. Sveiisl. Vetensh Akad. Ilandl., Bd. xii., 187-1. 

 - Ball. Sci. (Ill depart, du Nord, torn. viii. p. 113, 1870. 



» Monat.fher. d. k. Alcad. Wi.is. Bai'm, p. 561, 1877; and Zeitschr.f. iinss. /Cool, Bd. xxxiv. p. 144, 1880. 

 * Quart. Jovrn. Mia: Sci, N.S., vol. xxii. p. 387, 1882. 



■'' According to Langerhan.s there are ten of these segments in species of Oikopleura and FritiUaria he examined 

 at Madeira {Zcitschr. f. viss. Zool, Bd. xxxiv. p. 144). 



