Xll. INTRODUCTION. 



modification of ancient morphological characters. The result, 

 in my opinion, is a largely unnatural grouping. For example, 

 the well-marked group Thaliacea is broken up, and the 

 Doliolida3 are separated from their natural allies the Salpidas 

 and are united with the Distomidse. 



Sluiter's classification (1895) is much more natural, and 

 as to fundamental facts there is little difference of opinion 

 between us. So much so, that I am in hopes that he 

 may accept a modification which I shall here propose, and 

 which would bring us entirely into accord. In the primary 

 division into the three orders LARVACEA, THALIACEA and 

 ASCIDIACEA we are at one ; it is only in the further division 

 of the Ascidiacea that we differ, and that difference resolves 

 itself into the treatment of the Compound Ascidians. Here 

 again, as to the phylogeny or probable course of evolution of 

 the group we are agreed. As I pointed out at full length in 

 my " Challenger ' ; Keport,* and also expressed graphically 

 in a phylogenetic-tree diagram, the Compound Ascidians 

 must be regarded as a Polyphyletic group, and the families 

 Botryllida3 and Polystyelidse are probably derived from 

 CynthiidiE, while the other Compound Ascidians are more 

 nearly related to the Claveliuida3 and Ascidiidae. Still I 

 maintain that does not warrant us in separating in our 

 classification the Botryllida3 from the Distomida3 and uniting 

 them with the Cynthiidte as Sluiterf proposes. Whatever 

 their history has been in the past, the Botryllidre and the 

 DistomidaB are equally Compound Ascidians at the present 

 day. Both form colonies of Ascidiozooids produced by gem- 

 mation and imbedded in a common test, and consequently 

 I consider we are justified in uniting them as ASCIDIJE 

 COMPOSITE. 



On account, however, of the differences in origin, ex- 

 pressed in existing forms by certain differences of structure, 



* I'art II. USStJ), p. 3S7, &u. t Junaisuhe Denksehr. viii., p. 1(55. 



