508 



Mr. W. Garstang. On the 



[May 19, 



worthy of notice. They did not, however, draw any general conclu- 

 sions from the phenomena which they observed, beyond pointing out 

 that the irregular order in which the primary stigmata appeared 

 was in opposition to any theory as to their metameric arrange- 

 ment. 



The ouly other observations of importance are those of Seeliger* 

 on the development of the stigmata in Clavelina. In this form, as in 

 Phallusia and Molgula,f two pairs of stigmata at first arise, one behind 

 the other, near the dorsal border of the sides of the pharynx. But 

 instead of elongating in a transverse direction, as is the case in 

 Phallusia, these stigmata elongate in a longitudinal direction, and 

 become directly converted into the stigmata of the adult. With the 

 downward extension of the peribranchial chambers, new stigmata 

 arise independently, below the two first formed, so that eventually 

 two transverse rows of perforations are formed on each side of the 

 pharynx ; and all these, by growth in a longitudinal direction, become 

 directly converted into the slit-like stigmata of the adult. Sub- 

 sequently, after the attachment of the larva, new transverse rows of 

 stigmata arise in front of and behind the two first rows in an iden- 

 tical manner. 



I have myself followed out the development of the stigmata in 

 Clavelina, and have nothing to add to, or alter in, Seeliger 's descrip- 

 tion ; the stigmata invariably arise quite independently, and I have 

 seen no indication of such a process of subdivision as has been 

 described above for Phallusia (Ascidiella ?) scabro'ides. 



A similar independent mode of origin of the stigmata has also been 

 observed by Giard| in Perophora, and by Lahille§ in Distaplia magni- 

 larva. 



Thus, up to the present time, we are acquainted with three distinct 

 genera in which the stigmata arise independently of one another ; 

 while the process of subdivision, described for Phallusia (Ascidiella ?) 

 scabroides, remains unconfirmed and entirely without parallel. It 

 would even be excusable to regard this latter method, from its excep- 

 tional character, as a developmental modification of the former. But 

 before discussing this diversity of development, I will describe cer- 

 tain observations which I have made as to the development of the 

 stigmata in several other types of Ascidians. 



In Botryllus the stigmata of the adult have the usual form of 



* "Zur Entwicklungsgesch. d. Socialen Ascidien," 4 Jen. Zeit.,' vol. 18, 1885, 

 pp. 45—150, Plates 1 to 8. 



f P. J. van Beneden (M. ampulloides) . Kupffer, 'Arch. f. Mikr. Anat.,' vol. 8, 

 1872, Taf . 17, fig. 8a. Lacaze-Duthiers, * Arch, de Zool. Exp.,' vol. 3, 1874, pp. 

 623, 631, Plate 27. 



t ' Arch, de Zool. Exp.,' vol. 1, 1872, p. 677, Plate 24, fig. 6. 



§ ' Kecherches su: les Tuniciers,' p. 165. 



