TUNIOATA 



195 



are, therefore, not closely allied to the other Compound 

 Ascidiaus. The later Proto-Ascidiacea were probably 

 colonial forms, and gemmation was retained by the Clave- 

 liniilx and by the typical Compound Ascidians (Distomida, 

 &c.) derived from them. The power of forming colonies 

 by budding was lost, however, by the primitive Simple 

 Ascidians, and must, therefore, have been regained inde- 

 pendently by the ancestral forms of the Botryllidx and 

 the Polystyelidss. If this is a correct interpretation of the 

 course of evolution of the Tunicata, we arrive at the 

 following important conclusions. (1) The Tunicata, as a 

 whole, form a degenerate branch of the Proto-Chordata ; 

 (2) the Ascidix Saipiformes (Pyrosomd) are much more 

 closely related to the typical Compound Ascidians than 

 to the other pelagic Tunicata, viz., the Larvacea and 

 the Thaliacea ; and (3) the Ascidix Compositx form a 

 polyphyletic group, the sections of which have arisen 

 at several distinct points from the ancestral Simple 

 Ascidians. 



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(W. A. HE.) 



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