62 SCIENCE PROGRESS. 



is therefore retained throughout the life of the parent in 

 Doliolum, the Botryllidse, and the Polystyelidae. In the two 

 last families buds are produced on each side of the body of 

 the parent, a fact which indicates that the epicardial 

 diverticula were originally independent in function : each 

 provided the endoderm for the bud on the corresponding 

 side of the parent's body. The same conclusion may be drawn 

 from the formation of the single ventral bud of Distaplia, 

 since in this case only one of the two epicardial diverticula 

 which are present gives rise to the endoderm of the bud. 



In most cases, however, lateral budding has been replaced 

 by ventral budding, and each of the two epicardial outgrowths, 

 which were originally independent both in position and 

 function, contributes to the supply of endoderm in the median 

 ventral stolon. Doliolum, in my opinion, represents the most 

 primitive existing instance of this transformation of the epi- 

 cardial tubes ; in this form the epicardial tubes, although 

 both included in the stole prolifer, still retain their primitively 

 paired condition. Clavelina, on the other hand, seems to me 

 to represent a further advance in evolution ; for here the. 

 epicardial tubes, although separate in their origin, unite at 

 an early period to form an unpaired tube. Lastly, the final 

 stage in the transformation of the epicardial tubes is found 

 in Pyrosoma and Salpa, in which forms the tubes have lost 

 all trace of their primitively paired origin, and arise as a 

 median diverticulum of the pharynx, in the region of the 

 posterior cul-de-sac of the endostyle. 



It will be conceded, I think, that these modifications of 

 the epicardial tubes provide a sound basis for a true and 

 genetic classification of Tunicate budding ; and the subjoined 

 scheme is founded upon this basis. It should be premised, 

 however, that the first group within the scheme is marked 

 off from the rest by the entire absence of special epicardial 

 outgrowths. Budding in this group is effected in a much less 

 specialised manner than in any of the others ; in it the 

 endoderm of the bud is not derived from a single source but 

 from several, by the separate origin and eventual union of 

 pharyngeal and gastric portions. Without entering into a 

 special description of this type of budding — for which 



