324 M. Ussow’s Zoologico-Embryological Investigations. 
The complete absence of the cesophageal nervous ring which 
exists in the Mollusca, and indeed is characteristic of them, 
the unity in the structure of their central ganglion, and the 
development of all parts of their central nervous system from 
the upper germ-lamella in the form of a nervous ring becoming. 
segmented into three parts are facts which decidedly negative 
the homology erroneously ascribed to them (the so-called sipho- 
nal ganglion in Teredo navalis*). 
From a comparative revision of the nervous system in the 
different species of Tunicata, the following conclusions may 
be drawn :—in the Appendicularia the plan of structure of 
the nervous system is in some degree like that of the Ascidia ; 
the nervous system of the Pyrosomata may be regarded as a 
transition form between the transformed nervous system of 
the adult Ascidia, and the type of structure of the nervous 
system of the Salpe and Cyclomyarie. 
The process of transformation of the nervous system of the 
Ascidian larve commences immediately after their attachment 
(so-called sessile form). The ganglion is formed by multi- 
plication of the embryonal cells, which chiefly occupy the 
lower part of the upper sensory vesicle and the upper part of 
the trunk-vesicle. The caudal part (“dorsal cord ”’) of the 
embryonal nerve-tube is atrophied without leaving any traces. 
The pigment of the visual and auditory organs, and all other 
parts of the dissolving nervous system of the Ascidian larva, 
become converted into fat-drops, which are gradually absorbed 
by the young nerve-cells, with which the narrowing cavity 
of the nerve-vesicle is more and more filled. The formation 
of the blood-corpuscles is not dependent upon the above- 
mentioned metamorphosis of the vanishing embryonal system T. 
In this way the transformation of the embryonal nervous system 
into a central ganglion is effected. The ganglionic membrane 
is developed from the outer cell-layer of the young ganglion. 
At the time of the formation of the branchial fissures the 
ganglion is already almost completely developed. Numerous 
processes of cells of the nervous vesicle, which are at first 
spherical but gradually elongate and divide, gradually fill up 
its original cavity. The development of the peripheral nerves 
is effected by means of a chain-like coalescence of individual 
nerve-cells which occupy the inner mantle. The finely gran- 
ular protoplasm of these cells may be regarded as the original 
substance from which the fibrillar axial cylinders of the nerve- 
threads are formed. The stellate cells of the connective tissue 
form by coalescence the neurilemma of the above-mentioned 
nervous bundles. 
* Von Baer, loc. cit. p. 21. t+ Mull. Arch. 1852, p. 317, 
