STUDIES ON THE PROTOCHOEDATA. 299 



ally be supposed that tlie migration of the otocyst was an 

 active one, whereas, as we have seen, it is really passive. 



In Claveliua the migration of the otocyst is also effected by 

 relative growth, but occurs at a very early stage before the 

 cuticularisation of the wall of the cerebral vesicle (PI. 2", figs. 

 25 and 29—31). 



Meanwhile, in Ascidia mentula, the eye-cells collect 

 themselves together (figs. 4, 5), and finally, as is well known, 

 come to occupy the posterior right-hand corner of the cerebral 

 vesicle (fig. 2). 



After the sense-organs have taken up their definite positions, 

 the stomodseum forms by a median dorsal invagination of the 

 ectoderm (fig. 2), and shortly afterwards a communication is 

 established between the base of the stomodseum and the 

 branchial sac. At the stage at which the mouth breaks 

 through, Kowalevsky described the formation of an opening 

 between the cerebral vesicle and the stomodseum. I have 

 looked for this opening repeatedly in the tadpoles of Ciona 

 intestiualis, Phallusia mammillata, and Ascidia men- 

 tula, but have not been able to see it; and, in fact, I will go 

 so far as to say, in confirmation of Kupfi'er (8), that the open- 

 ing as described by Kowalevsky does not exist in the tailed 

 larvse. 



At a later stage, after the commencement of the metamor- 

 phosis, a corresponding opening is actually formed, the relations 

 of which are essentially the same as those of the pore described 

 by Kowalevsky, though with certain important difi'erences ; but, 

 nevertheless, Kowalevsky's actual observation was, according 

 to my account, erroneous. Thus in the tadpoles of the above- 

 named simple Ascidians there is no communication what- 

 ever between the cavity of the cerebral vesicle and the stomo- 

 dseum. 



On the other hand, as we shall see, in the tadpole of Clave- 

 lina there is such a communication, which was, however, denied 

 by van Beneden and Julin. We have, therefore, the curious 

 circumstance that what Kowalevsky asserted in the case of 

 Phallusia mammillata was contradicted by van Beneden 



