323 ^.JJ&&oyi^sZcH>logico-Embryolo<jical Investigations. 



h, paired, without canals {Pi/rosoma^) ; and c, paired, and 

 furnished with two canals (8alpid;i?t). The position of the 

 auditory vesicles is very dift'erent in diflerent species of Tuni- 

 cata. They are often situated in the neighljourhood of the 

 central ganglion {^ipjiendicuhuiuj Pi/rosonutj Saljiidaj), and are 

 always united either with a special nerve [nervus ncuiiticu.s) 

 which terminates in their thin walls, or with a short ])edunelc 

 of the ganglion {Apj>cn(/iculariaj Pijrosoma). In the Sali)ida!, 

 in which they have the form of shallow funnels, the aiutitory 

 vesicles are closely applied to the ganglion by their base, 

 whilst the spirally twisted canals issuing from their a])ex 

 open by wide aj)ertures into the branchial cylinder. Within 

 the auditory vesicles are lined with simple epithelium, in 

 which no bacillar processes are perceptible. Tlie number of 

 shining calcareous otoliths, which are sometimes coloured 

 (Pt/rosonia), enclosed both in the auditory vesicles themselves 

 and in their canals (Salpidffi) is very various : in the Appendi- 

 cukirice and Cydomyariw there is usually only one, wiiilst in 

 Pi/rosoma, and especially in the Salpidaj, their number is very 

 considerable. To my great regret, I am unacquainted with 

 the development of the auditory vesicles. 



4. Visual organs. — These 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 J {Salpa, Pi/rosonia). 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 j)art of the nervous 

 system. The simple eyes of the Ascidia [Ascidia intestinalis^ 

 mentuhj canina, &c.) are very numerous (8/6). In the Pyro- 

 somata and many groups of Salpie the eye is usually unpaired 

 (Salpa fusiformisy africana — majcima^ democratica — mucro- 

 nata) ; in the rest it is paired {Salpa hicaudatn), and even 

 triple {Salpa jyinnata). The outer surface of the eye is turned 

 sometimes toAvards the respiratory or anterior orifice {Ascidia^ 

 many Salpa) y sometimes towards the cloacal or posterior 



* Whilst one, in Pyrosoma f/if/a.i, lies beneath the central panglion, 

 the other occurs on the inner surface of the tubular lip of the anterior 

 orifice. 



t See 11. Miiller's description, Zeitschr. fiir wiss. Zool. p. 330; Leuckart, 

 loc. cit. Ileft ii. p. 25. 



X Afl described by Kowalevsky in »Sfl^a (Gotting, Nachr. 1868, p. 410). 



