326 M. Ussow’s Zoologico-Embryological Investigations. 
b, paired, without canals (Pyrosoma*); and c, paired, and 
furnished with two canals (Salpidet). The position of the 
auditory vesicles is very different in different species of Tuni- 
cata. ‘They are often situated in the neighbourhood of the 
central ganglion (Appendicularia, Pyrosoma, Salpide), and are 
always united either with a special nerve (nervus acusticus) 
which terminates in their thin walls, or with a short peduncle 
of the ganglion (Appendicularia, Pyrosoma). In the palpee, 
in which they have the form of shallow funnels, the auditory 
vesicles are closely applied to the ganglion by their base, 
whilst the spirally twisted canals issuing from their apex 
open by wide apertures into the branchial cylinder. Within 
the auditory vesicles are lined with simple epithelium, in 
which no bacillar processes are perceptible. The number of 
shining calcareous otoliths, which are sometimes coloured 
(Pyrosoma), enclosed both in the auditory vesicles themselves 
and in their canals (Salpidee) is very various : in the Appendi- 
cularte and Cyclomyarie there is usually only one, whilst in 
Pyrosoma, and especially in the Salpide, their number is very 
considerable. To my great regret, 1 am unacquainted with 
the development of the auditory vesicles. 
4, Visual organs.—T hese organs are developed in the Tuni- 
cata, either by a depression of the epithelial layer of the inner 
mantle (ocelli of the simple and social Ascidia), or by the 
anterior wall of the upper vesicle of the embryonal nerve- 
tube being pushed out} (Salpa, Pyrosoma). They make their 
appearance very late in all Ascidia; but in the sedentary 
Tunicata they are to be seen already in the embryonal state. 
The pigment of the visual organs, which at first consists of 
round and slightly coloured, and subsequently of hexagonal 
united cells, is developed from the same embryonal cells of 
the outer layer of the above-mentioned part of the nervous 
system. The simple eyes of the Ascidia (Ascidia intestinalis, 
mentula, canina, &c.) are very numerous (8/6). In the Pyro- 
somata and many groups of Salpe the eye is usually unpaired 
(Salpa fusiformis, africana—maxima, democratica—mucro- 
nata); in the rest it is paired (Salpa bicaudata), and even 
triple (Salpa pinnata). The outer surface of the eye is turned 
sometimes towards the respiratory or anterior orifice (Ascidia, 
many Salpe), sometimes towards the cloacal or posterior 
* Whilst one, in Pyrosoma gigas, lies beneath the central ganglion, 
the other occurs on the inner surface of the tubular lip of the anterior 
orifice. 
+ See H. Miiller’s description, Zeitschr. fiir wiss. Zool. p. 830; Leuckart, 
loc. eit. Heft ii. p. 25. 
t As described by Kowaleysky in Sa/pa (Gotting. Nachr. 1868, p. 410). 
