M. Ussow's Zoologico-Emhryohgical Investigations. 325 



II. 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 multi})olar cells (of induhitable 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. 



h. More composite tactile organs of the Tunicata are bacilli- 

 form, acutely pointed processesf of similar but rather smaller 

 peripheral nerve-cells, sometimes uniting in groups (Doliolidaa). 

 These processes occur in the lips and some other parts of the 

 inner mantle in some species of natatory Tunicata (Salpidte, 

 Doliolidae) . 



2. Olfactory organs. — The so-called ciliated pit of un- 

 doubtedly nervous nature, which is not unfrequcntly com- 

 bined with a special nerve {nervus olfactorius — Saljya, Dolio- 

 lum, Fyrosonia, &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 

 Salpidai), the walls of which then become repeatedly folded, 

 and thus form more or less numerous curved vibratile cavities 

 (in most of the sedentary Tunicata and many genera of Salpidaj). 

 In Ascidia mammillata the number of simple ciliated cavities 

 rises to two hundred, which are united among themselves by 

 means of ramifying cffical 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 suiTOunded by peculiar spherical pigment- 

 cells, the number of which appears to increase with the age of 

 the animal (especially in Ascidia mammillata). 



3. Auditory organs. — The so-called auditory vesicles 

 occur : — a, unpaired, singly {Aj)2>e>idicula7-ice, Cyclomyarice) j 



* 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. 32o ; see also Boll, Beitr. zur verprl. Ilistol. p. 20. 



t As in other Mollusca. See Leydig, Lehrb, der Histol. p. 212 ; 

 Schultze's Archiv, p. 448, Taf. 2o, fig. 6. 



Ann. & Mag. N. Hist. Ser.4. VoI.kv. 23 



