M. Ussow's Zoologico-Evihryological Investigations. 327 



opening {Pyrosoma) j and with paired or triple eyes towards 

 both openings. In all f2;roups of tlic Sdlpir. and Pgrosoiiuitd the 

 eyes arc united to the central ganglion by means of a peduncle 

 of greater or less length (nervus opticus) • while in theAscidia, 

 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, Pgrosomu)^ or destitute of these 

 so-called eye-chambers {Saljxc). In the latter case the clavatc 

 exti'cmitics 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 Pgrosoma) contain also a verti- 

 cal transparent lenticular body composed of concentric layers 

 (just as €. g. in 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 Appendiculariidae 

 and Doliolidajf, which display no trace of eyes) we meet with 

 different t}"]jes of structm-e. 



While the so-called "ocelli" of the Ascidia represent the 

 eyes of the lower Crustacea and Vermes, the compound eyes 

 of the Salpce are homologous witli the visual organs of the 

 Arthropoda ; the single eye of the Pyrosomata^ which is fur- 

 nished Avitli 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 veiy 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 

 sensoiy organs in the natatoiy Tunicata, may be explained 

 by the more stin'ing and energetic mode of life of these 

 animals. 



* F. AVill (Froriep's Notiz. 1844) found a flat lens in such Ascidian 

 eves ; but this is not confirmed bj my investigations. See also Bronn, 

 Ueicbth. Abth. i. p. lo4. 



t In iJoliuhnu denticidattmi, Nordm., &c. I often foimd behind the 

 ganglion a simple n-d aggregation of pigment (sometimes also in some 

 BotrylU), •which, however, cannot be bkeued to the so-called ocelli. 



^3* 



