1892.] Development of the Stigmata in Aseidians. 



509 



longitudinally elongated slits, arranged in a series of transverse rows 

 on each side of the pharynx. 



w Various points in the development of Botryllus have been elucidated 

 by the researches of Metschnikoff,* Krohn,f and Ganin,£ but the 

 development of the stigmata remains still un described. 



The stigmata of the oozooid arise in a manner very different from 

 that which we have seen in the case of the compound Aseidians 

 Clavelina, Perophora, and Distaplia ; the mode of their development 

 recalls the phenomena described by E. van Beneden and Julin for 

 " Phallusia " scabroides, but presents distinctive features of consider- 

 able importance. 



In the earliest stage which has come under my observation, the 

 young zooid (B. aurolineatus, Giard) is already fixed and is provided 

 with the rudiments of two buds, one on each side. The endodermic 

 vesicles of the buds as yet show no signs of differentiati m. The zooid 

 itself possesses the rudiments of two lateral tentacles only, and is pro- 

 vided with the eight club-shaped ectodermic processes, with long 

 stalks, which are so characteristic of the larva. 



The pharynx is provided with four pairs of transversely elongated 

 stigmata, whose transverse diameters are nine times as great as their 

 antero-posterior diameters. These huge transverse slits, whose width 

 almost equals the length of the endostyle, extend right across the 

 sides of the pharynx, from the dorsal region to the endostyle. They 

 are not exactly of equal size, but decrease slightly in width in regular 

 order from before backwards. The second slit is 0*2 mm. wide. The 

 endostyle at this stage is 0*25 mm. long. 



In the next stage examined (P. aurolineatus) the endodermic 

 vesicle of each bud is already differentiated into a median pharyngeal 

 portion and a pair of lateral peribranchial portions. The oozooid has 

 now the rudiments of four tentacles, and the number of ectodermic 

 processes has increased to eleven, the separate stalks being now very 

 short. 



The pharynx possesses, in place of the four pairs of transversely 

 elongated stigmata, four transverse rows of small stigmata on each 

 side. The anterior row is 0*42 mm. wide, the second row is 35 mm. 

 wide, and the two posterior rows are still narrower. The endostyle 

 at this stage is 0*425 mm. long. The first row consists of 10 stigmata, 

 the second of 8 stigmata, the third of 6 or 7, the fourth of still fewer. 

 None of the stigmata are elongated transversely ; they are, for the 

 most part, of an oval form, slightly elongated longitudinally, but 

 towards the dorsal side they are more or less circular. 



At a still more advanced stage (sp. incert.) the oozooid has attained 



* ' Bull. Acad. Ii-np. Sci. St. Petersbourg,' 1869, pp. 291—293. 

 f ' Arch, f . Naturgesch.,' vol. 35, 1869, pp. 190—196, 326—333. 

 t " Neue Thatsachen ; " c Zeit. f. Wise. Zool.,' vol. 20, 1870. 



2 m 2 



