REPORT OX THE TUNICATA. 149 



from wliicli he derives (1) Pyrosoma through DistapUa, (2) the BotrylHchx!, and (3) 

 the Social Ascidians through the Diplosomidaj aud the Didemuidse. 



There are several points in that scheme which I cannot agree with. It seems to me 

 that the passage from Appendicularia mossi through Anchinia rubra to Doliolum, and 

 through the ancestral Doliolidge to Salpa, is so natural and simple that it l)ecomes very 

 improbable that the Thaliacea have ever been fixed forms. It is extremely unlikely 

 that they are, as Uljanin supposes, a group of Simple Ascidians, which after Ijeing 

 fixed betook themselves again to a free-swimming mode of life aud underwent great 

 modification. 



I think it is more probable that the Simple and the Compound Ascidians 

 were l:)oth derived from a common ancestor, than that the Compound were evolved 

 from the Simple ; and I object strongly to Uljanin's view, that the Social Ascidians 

 are a group derived from the Compound forms and having no close connection 

 with the Simple Ascidians. This is opposed to all we know as to the very close 

 relationship * between the Clavelinidse and the Ascidiidse. There can, I think, be no 

 doubt after the examination of such a series of forms as Diazona, Clavelina, 

 Ectehiascidia, and Ciona, that the Social Ascidians (Clavelinidse) are intermediate 

 between the least modified of the Simple Ascidians aud the least modified of the 

 Compound Ascidians, and ought therefore to be regarded as closely allied to the 

 ancestral form from which both Simple and Compound Ascidians were derived (see E. 

 in table, p. 150). 



Finally, I may point out the two most important conclusions at which I have arrived, 

 as the result of this investigation into the Phylogeuy of the Tunicata : — 



1st, Pyrosoma, although now a pelagic free-swimming organism, was derived from the 

 fixed Compound Ascidians. The discovery of Coslocormics huxleyi shows the relationship 

 between Pyrosoma and the primitive Didemuida3, and the latter were derived from the 

 primitive Distomidse ; consec^uently Pyrosoma is directly related to the most typical of 

 the Compound Ascidians. 



2ud, The Ascidi^e Compositse or Synascidia; are polyphyletic, having Ijeen derived 

 from the Simple Ascidians or their ancestors at three distinct points. The result of 

 this is that the Compound Ascidians form three groups (see table, p. 150), — (l) the 

 Polystyelidas, (2) the Botryllidas, and (3) the remainder, which are more nearly related 

 to particular groups of Simple Ascidians than they are to one another. 



1 See this Report, P;irt I. pp. 237 ct scq.; and Sluiter, Natuurkutul. Tijilschi: v. Ncdcrt. Indoj, Dl. xlv. p. 100, 

 1886. 



