UROCHORDA. 37 



rations known amongst the Ascidians is that in Doliolum. The 

 discovery of this metamorphosis was made by Gegenbaur (No. 

 10). The sexual form of Doliolum is somewhat cask-shaped, 

 with ring-like muscular bands, and the oral and atrial apertures 

 placed at opposite ends of the cask. The number of gill slits 

 varies according to the species. The ovum gives rise, as already 

 described, to a tailed embryo which subsequently develops into 

 a cask-shaped asexual form. On attaining its full size it loses 

 its branchial sack and alimentary tract. While still in the 

 embryonic condition, a stolon grows out from its dorsal side in 

 the seventh inter-muscular space. The stolon, like that in Salpa, 

 contains a prolongation of the branchial sack 1 . 



On this stolon there develop two entirely different types of 

 buds, (i) lateral buds, (2) dorsal median buds. 



The lateral buds are developed in regular order on the two 

 sides of the stolon, and the most advanced buds are those 

 furthest removed from the base. They give rise to forms with a 

 very different organization to that of the parent. They are 

 compared by Gegenbaur to a spoon, the bowl of which is formed 

 by the branchial sack, and the handle by the stalk attaching the 

 bud to the stolon. The oral opening into the branchial sack is 

 directed upwards: an atrial opening is remarkably enough not 

 present. The branchial sack is perforated by numerous open- 

 ings. It leads into an alimentary tract which opens directly to 

 the exterior by an anus opposite the mouth. 



The stalks attaching the more mature buds to the stolon are 

 provided with ventrally directed scales, which completely hide 

 the stolon in a view from the ventral surface. 



Thes buds have, even after their detachment, no trace of 

 generative organs, and shew no signs of reproducing themselves 

 by budding. Their eventual fate is unknown. 



The median dorsal buds have no such regular arrangement 

 as the lateral buds, but arise in irregular bunches, those furthest 

 removed from the base of the stolon being however the oldest. 

 These buds are almost exactly similar to the original sexual 

 form ; they do not acquire sexual organs, but are provided with 



1 1 draw this conclusion from Gegenbaur's fig. (No. 10), PI. xvi., fig. 15. The 

 body (x) in the figure appears to me without doubt the rudiment of the stolon, and 

 not, as believed by Gegenbaur, the larval tail. 



