M. Ussow’s Zoologico-Embryological Investigations. 323 
centre is occupied by their processes, which run in the direc- 
tion of the longitudinal axis of the ganglion. 
All the nervous elements, except for their small size 
(0:003-0°02 millim.) and the complete absence of the so-called 
sheath (Markscheide), differ but 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 cesophageal nervous ring which has been de- 
scribed by some naturalists (Delle Chiaje*, Eschscholtzt, &c.) 
is wanting in all the Tunicata examined by me. The number 
of peripheral nerves developed independently of the ganglion 
is very different in different species, groups, and generations 
(Salpe). It varies between three single nerves (Cynthia 
papilosa) and thirty-six pairs of nerves (Salpa maxima, 
pinnata, bicaudata, &e.). Peripheral ganglia occur in the 
Appendicularie, 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 Appendicularie, 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 Appendicularie and 
that of the embryo and larva of the Ascidia is presented by 
the division of their central ganglion into three parts, which 
are particularly observable in Appendicularia flagellum. The 
central ganglion of this animal is divided into :—1, an upper 
conical part, with 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 Cynthie (C. microcosmus). The 
nervous system of the 'Tunicata in the retrograde state can by 
no means be compared with the nervous system of the Mol- 
lusca (Baer), either with regard to the morphological plan of 
its structure or, still more, as respects the type of its embryonal 
development. 
* Notom. degli Anim. Invert. vol. iii. pp. 28, 29. 
+ Isis, 1824, p. 5. 
{~ Mull. Arch. 1846, p. 106; Leuckart, Zool. Unters. Heft ii. p. 85; 
Phil. Trans. 1851, p. 596, tab. xviii. fig. 22: Kowalevsky, KEntwicke- 
lungsgesch. der einf. Ascid. p. 13; Kowalevsky, Kiew Zapiski, vol. iii. 
part 1. p. 47. 
