322 M. Ussow’s Zoologico-Embryological Investigations. 
the Tunicata, and find in them sharply defined characters of * 
the molluscan type. 
In my investigations of the Tunicata the task I set myself 
was, in the first place, to test by personal investigation the 
exceedingly important results of their developmental history, 
and, secondly, to clear up as far as possible some gaps and 
disputed questions in their anatomy. In the latter direction 
I endeavoured chiefly to investigate :—1, the structure of the 
nervous system and its mode of transformation from the form 
proper to the embryonal and larval states, which have hitherto 
been very superficially observed and described ; 2, the intimate 
structure of all the sensory organs of the Tunicata, which has 
not yet been satisfactorily ascertained ; and, 3, the structure of 
the inner and especially of the outer mantle, and of the organs 
of circulation and nutrition, which likewise presented important 
questions still unsolved. 
The following forms were investigated by me in different 
directions :— 
I. Sepentary Tunicata :—Ascidia mammillata, intesti- 
nalis, canina, mentula; Cynthia microcosmus, papillosa, 
ampuloidea ; Clavellina lepadiformis; Botryllus smaragdus, 
auratus ; Diazone violacea. 
Il. Swimmine Tunicata :—Appendicularia furcata, fla- 
gellum, cerulescens ; Pyrosoma gigas; Salpa africana—maz- 
ima, democratica—mucronata, runcinata—fusiformis, bicau- 
data, pinnata; Doliolum, Ehrenb., Nordm., Miill. 
I now proceed to give a condensed statement of the results 
of my investigations. 
I. The Nervous System—its anatomy, minute structure, 
and mode of formation. 
All the Tunicata investigated by me (except the Appendi- 
cularia) have a single unpaired ganglion (central ganglion of 
authors, 0°1—-0°15 millim. in diameter), which is both analogous 
and homologous with the central portion of the nervous system 
of the lower Vertebrata (Amphioxvus). It is always situated 
in the middle line on the dorsal side of the animal, close to 
or not far from the entrance of the respiratory cavity. Both 
the ganglion and all the peripheral nerves occur in the trans- 
parent layer of the inner mantle, in which the muscular bundles 
and the reticulately fibrous connective tissue (dermo-muscular 
sac) are also imbedded. ‘The distribution of the nervous 
elements in the ganglion is very simple and uniform. The 
usually multipolar nerve-cells (gymnocyta) are generally placed 
in layers in the peripheral part of the ganglion, whilst its 
