M. Ussow’s Zoologico-Embryological Investigations, 327 
opening (Pyrosoma), and with paired or triple eyes towards 
both openings. In all groups of the Salpe and Pyrosomata the 
eyes are united to the central ganglion by means of a peduncle 
of greater or less length (nervus opticus); while in the Ascidia, 
in which their distance from the ganglion is considerable, the 
union is effected by thin ramifications of the anterior pair of 
nerves. ‘The eyes, which are usually of a more or less oval 
form, are either furnished with internal cavities* filled with a 
transparent substance (Ascidia, Pyrosoma), or destitute of these 
so-called eye-chambers (Sa/pe). In the latter case the clavate 
extremities of rather long bacilliform processes of the nerve- 
sheath, which fill the whole eye, form a hemisphere. The 
eyes furnished with chambers have, besides the attenuated 
epithelial layer of the inner mantle which covers them, a thin 
proper sheath (Ascidia), or (in Pyrosoma) contain also a verti- 
cal transparent lenticular body composed of concentric layers 
(just as e.g. im Anodonta). When the eye-chambers are 
wanting, the structure of the eyes is similar to that of the 
Insects and higher Crustacea ; in other words, it approximates 
to the type of the compound faceted eyes. 
Thus in the mode of development and structure of the 
visual organs in the 'Tunicata (except the Appendiculariide 
and Doliolidet, which display no trace of eyes) we meet with 
different types of structure. 
While the so-called “ocelli” of the Ascidia represent the 
eyes of the lower Crustacea and Vermes, the compound eyes 
of the Salpe are homologous with the visual organs of the 
Arthropoda ; the single eye of the Pyrosomata, which is fur- 
nished with a lens, may even be likened to the visual organs 
of some Mollusca. The fact that, when compared with the 
sedentary 'Tunicata, the natatory forms with an equal or even 
smaller size of the central ganglion possess a greater number 
of peripheral nerves, depends very probably upon the greater 
development of their locomotive organs (annular or ribbon- 
like muscles). The great number and high degree of deve- 
lopment of these, and the much greater development of the 
sensory organs in the natatory Tunicata, may be explained 
by the more stirrmg and energetic mode of life of these 
anunals. 
* F. Will (Froriep’s Notiz. 1844) found a flat lens in such Ascidian 
eyes; but this is not confirmed by my investigations. See also Bronn, 
Weichth. Abth. i. p. 154. 
+ In Doliolum denticulatum, Nordm., &c. I often found behind the 
ganglion a simple red aggregation of pigment (sometimes also in some 
Botrylli), which, however, cannot be likened to the so-called ocelli. 
23* 
