BUDDING IN TUNIC AT A. 61 



the free ends of the epicardial diverticula unite with one 

 another to form an unpaired tube, the cavity of which 

 becomes practically obliterated by the juxtaposition of its 

 walls. This unpaired tube then penetrates the stolon, and 

 usually presents the appearance of a thin broad band 

 dividing the cavity of the stolon longitudinally into two 

 separate blood -channels ; under this form it has long been 

 known as the stolonial septum, but its true nature and 

 origin were first elucidated by Van Beneden and Julin. 

 The fusion of the epicardial tubes in Clavelina and the 

 majority of the fixed Ascidians should not, however, 

 disguise the fact of their essentially paired nature. In 

 Doliolum, as I have shown above, this primitive condition 

 is retained ; and in Distaplia, as Salensky's recent investi- 

 gation has revealed, they are still distinct from one another 

 at the time when the first (or ventral) bud is produced. In 

 Botryllus also the homologues of the epicardial outgrowths 

 are paired throughout life. In this form, however, they 

 communicate with the pharynx only in their earliest stages, 

 while the peribranchial diverticula are still in open com- 

 munication with the pharynx in the same region. At a 

 later stage the peribranchial sac and epicardium of each 

 side become separated from the pharynx together ; and in 

 the full-grown organism the epicardial tube is a capacious 

 diverticulum of the peribranchial sac. In spite, therefore, 

 of the final difference of position between the epicardial 

 (or perivisceral) sacs of the Botryllidae and the epicardial 

 tubes of Distaplia or Clavelina, there can be no doubt, as 

 Pizon has maintained, that there is an exact homology 

 between the two structures. In the buds of Distaplia, 

 according to Salensky, the epicardial sacs show a similar 

 connection with the peribranchial diverticula in the earliest 

 stages of their formation ; but, at a later period, the peri- 

 branchial sacs separate completely from the pharynx, while 

 the epicardial diverticula retain their connection with it. 



It appears also, from a recent paper by Ritter (21), that 

 Goodsiria, one of the Polystyelidae, presents the same 

 relations as Botryllus. 



The primitively paired form of the epicardial diverticula 



