M. Ussow’s Zoologico-Embryological Investigations. 325 
Il. The structure and mode of formation of the Sensory 
Organs. 
1. The tactile nervous apparatus which are met with in all 
Tunicata may be classified as follows, in accordance with the 
peculiarities of their structure :— 
a. Simple, very uniformly constructed apparatus. The 
peripheral multipolar cells (of indubitable nervous nature) 
united with the thin terminal ramifications of the nerves* 
emit numerous processes, which unite directly with the pro- 
toplasm of the epithelial cells (“nerve-epithelium’’) of the 
inner mantle. 
b. More composite tactile organs of the Tunicata are bacilli- 
form, acutely pointed processes} of similar but rather smaller 
peripheral nerve-cells, sometimes uniting in groups (Doliolide). 
These processes occur in the lips and some other parts of the 
inner mantle in some species of natatory Tunicata (Salpide, 
Doliolide). 
2. Olfactory organs.—The so-called ciliated pit of un-' 
doubtedly nervous nature, which is not unfrequently com- 
bined with a special nerve (xervus olfactorius—Salpa, Dolio- 
lum, Pyrosoma, &c.), is developed in the form of a depression 
of the epithelial layer (of the upper germ-lamella) of the inner 
mantle. At first it contains only one cavity (which persists 
throughout life in Doliolum, Pyrosoma, and some genera of 
Salpidee), the walls of which then become repeatedly folded, 
and thus form more or less numerous curved vibratile cavities 
(in mostof the sedentary Tunicata and many genera of Salpide). 
In Ascidia mammillata the number of simple ciliated cavities 
rises to two hundred, which are united among themselves by 
means of ramifying cecal ciliated tubes situated, like the 
cavities, in the transparent middle layer of the inner mantle. 
In the Ascidia just mentioned the openings of the ciliated pits 
occur in the internal space (atrial chamber, Huxley), between 
the inner epithelial layer and the wall of the branchial sac. 
In the cavities there is always only a one-layered vibratile epi- 
thelium, sometimes surrounded by peculiar spherical pigment- 
cells, the number of which appears to increase with the age of 
the animal (especially in Ascedia mammillata). 
3. Auditory organs.—The so-called auditory vesicles 
occur :—a, unpaired, singly (Appendicularie, Cyclomyarie) ; 
* Similar cells are mentioned by Leuckart, Zool. Unters. Heft ii. 
p- 23, as also in the Heteropoda and other Mollusca, Zeitschr. fiir wiss. 
Zool. iv. p. 825; see also Boll, Beitr. zur vergl. Histol. p. 20. 
+ As in other Mollusca. See Leydig, Lehrb. der Histol. p. 212; 
Schultze’s Archiv, p. 448, Taf. 25. fig. 6. 
Ann. & Mag. N. Hist. Ser.4. Vol. xv. 23 
