M. Ussow's Z(iol(>(/ico-Knifift/<'/(>i/i,fi/ l/ircslii/dfid/is. '.'r2'.> 



centre is occupicil by their [)rocesse.s, which run in the direc- 

 tion of the loniiitmlinal axis of the ^an^lioii. 



All the nervous elements, except for their small size, 

 (0"00.'!-()M)2 millim.) and the comjilete absence of theso-eaUed 

 sheath [M<irkscJiri<U')^ differ hut little from the elements of the 

 nervous tissue which occur, for example, in the cerebellum, 

 the Hasserian ganglion, and in other parts of the central 

 nervous system of the Vertebrata (especially the Fishes). 

 The so-called opso})hageal nervous ring which has been de- 

 scribed by some naturalists (Delle Chiaje*, Eschscholtzf, &.c.) 

 is wanting in all the Tunicata examined by me. The num1)er 

 of periplieral nerves develoj)ed inde])endently of the ganglion 

 is very different in different s])ecies, groups, and generations 

 {Salpa'). It varies between three single nerves {Cynthia 

 papillosa) and thirty-six pairs of nerves {Salpa maxiina, 

 pinnata, bicaudata, Sec). Peripheral ganglia occur in the 

 Appetulicularue, while in all other Tunicata no such ganglia 

 are met with either in the embryonic or in the fully-developed 

 state. The ganglia caudalia of the Appendicularia;^ from ten 

 to eighteen in number, which are united by means of an in- 

 ferior nerve of the central ganglion, form a chain \ extending 

 into the tail, running over the so-called axial cord (like the 

 chorda dorsalis). A something in common in the plan of 

 structure of the nervous system of the Append Iculariai and 

 that of the embryo and larva of the Ascidia is presented by 

 the division of their central ganglion into three parts, whicli 

 are [particularly observable in App)endicularia jJagelluin. The 

 central ganglion of this animal is divided into : — 1, an upper 

 conical part, Avith three pairs of nerves; 2, a middle, spherical 

 part, with the auditory vesicles seated upon it ; and, 3, a lower 

 wedge-shaped part, with two paired nerves and an inferior 

 unpaired nerve, the latter forming as it were the continuation 

 of the ganglion and extending to the extremity of the tail. 

 We find a similar division of the central ganglion (sometimes 

 with a trace of the central cavity or "central canal") in very 

 young fixed Ascidia, e.g. the Cynthice (C. mierocosimis). The 

 nervous system of the Tunicata in the retrograde state can by 

 no means be compared with the nervous system of the j\Iol- 

 lusca (Baer), either with regard to the moi-phological plan of 

 its structure or, still more, as respects the type of its embryonal 

 development. 



• Notom. degli Anim. Invert, vol. iii. ])n. 2S, 20. 



t Isis, 18-24, p. o. 



t Miill. Arch. 1846, p. 100 ; T.oiickart, Zool. Tntoi-i?. Iloft ii. p. So : 

 Phil. Tran.<!. 1851, p. 5iKl, tab. xviii. tier. 2m\ Kowolev.sky, EutwirKe- 

 liinfr-^tresch. cIpt eini. A.«cifl. p. I'l; Kowalev.^ky, Kiew Zapiski, vol. iii. 

 part 1. p. 47. 



