INTRODUCTORY NOTE 



( This paper consists of two chapters, VII and VIII, printed, in 

 advance, from my memoir on the Genus Salpa, which is now passing 

 through the press.) 



In chapter VI of ray memoir on the Genus Salpa I shall give, at 

 length, reasons which, briefly stated, are as follows, for believing 

 that, while salpa is remotely descended from a pelagic, append icu- 

 laria-like ancestor, it is more immediately derived from a sessile 

 form, similar in its habit of life, and essentially, in structure also 

 to the ascidians. 



In the first place, comparative anatomy forces us to believe that 

 the atrium of salpa is identical with the perithoracic and atrial 

 chamber of ordinary ascidians, and the facts of embryology show 

 beyond question that this is a real homology. 



Writers on the embryology of salpa and allied animals have 

 involved the history of the atrial system in unnatural obscurity, 

 for its origin in the salpa embryo and in the aggregated salpa 

 is in perfect accordance with the teaching of comparative anatomy, 

 and quite irreconcilable with any view except the one which re- 

 gards these structures in salpa as strictly homologous with the 

 median and lateral atria of ordinary ascidians. 



As Leuckart pointed out long ago, the atrial aperture of salpa 

 is much nearer the mouth when it first appears than it is later, 

 and in this respect the ontogeny of salpa exhibits evidence of an 

 ascidian-like stage in its ancestry. The compactness of the ganglion 

 of salpa, as contrasted with the elongated central nervous system 

 of primitive chordata, and its position between the two apertures 

 of the body, are also features of resemblance to the ascidians ; and 

 while there are now no traces, at any stage of its development, of 

 numerous stigmatic gill-slits, like those of pyrosoma, there is ample 



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