518 TUNICATA. 



however, say that in Pyrosoma, as well as in the Salpidae, the 

 embryonic development has undergone great coenogenetic variation. 

 In Pyrosoma, the accumulation of food-yolk, and, in Sal pa, the fusion 

 of the embryo with the body of the parent as well as other conditions 

 have brought about such variations. In both cases, development is 

 direct, and, as compared with the development of the Ascidians, much 

 abbreviated. The suppression in the embryonic development of the 

 Salpidae of so important an organ as the chorda dorsalis is of interest. 

 In the Cyathozooids of the Pyrosoma, also, this organ does not develop 

 distinctly, although SALENSKY thought that he found a trace of its 

 rudiment. The chorda-rudiment is also wanting in the buds of all 

 Tunicates. SALENSKY, indeed, regarded the elaeoblast, that pro- 

 visional organ of the Salp embryo, as the rudiment of the chorda, 

 but the fact that the elaeoblast also occurs in the buds of Pyrosoma 

 and Salpa, while other provisional larval organs are not found as 

 rudiments in these buds, throws doubt upon this view. It is found, 

 for example, that the nervous system in the second individuals pro- 

 duced by budding in the larval Diplosoma no longer show the larval 

 character (p. 459). The elaeoblast is wanting in the buds of the 

 composite Ascidians and the ^Doliolidae, nor does its rudiment appear 

 in the Cyathozooids of Pyrosoma. 



SALENSKY maintains that, in the chief groups of the Tunicata, the 

 follicle-cells take part in the formation of the embryo. We are 

 somewhat sceptical as to these statements, which will be found more 

 in detail on pp. 357, 392 and 421 and think that this supposed 

 participation may be reduced to the absorption of the follicle-cells as 

 nourishment by the blastomeres [see footnote, pp. 420 and 421]. 



It is evident from the above that, in judging of the systematic 

 position and phylogeny of the Tunicates, we are dependent almost 

 entirely on the embryonic development and metamorphosis of the 

 Ascidiacea. Among these, the solitary forms and Clavelina have 

 yielded the most valuable material, while the eggs of the composite 

 forms, which are rich in yolk, show a derived condition. 



Among the Tunicates now living, Appendicularia is regarded as 

 showing in its organisation the most primitive conditions. We must, 

 however, raise the question as to how far these conditions are really 

 primitive. The Larvacea show remarkable resemblance in their 

 structure to the free-swimming tailed Ascidian larvae, from which 

 they are distinguished chiefly by the absence of a common atrial 

 cavity, the anus and the two peribranchial tubes opening out 

 independently on the ventral side of the body. Since the abbrevia- 



