350 ARTHUR WILLEY. 



atrial expansions. But, as we have seen above, the atrial 

 cavities of the Ascidians are special structures, and not merely 

 parts of a primary pair of gill-slits, just as is the atrial 

 chamber of Amphioxus, with which, as I have shown, the former 

 are probably homologous. 



The condition of the atrial cavities in Appendicularia points 

 therefore, rather to a reduction from a more highly developed 

 condition. There are many other points in the anatomy of 

 Appendicularia which show a reduced rather than a primitive 

 character. For instance, the condition of the hypophysis, con- 

 sisting as it does of a simple funnel-shaped pit extending from 

 the pharynx to the base of the brain, is an undeniable indica- 

 tion of secondary reduction which will be appreciated from an 

 account of the nervous system, which will form the second of 

 " these Studies," and is closely paralleled by what occurs in 

 many compound Ascidians, where the reduction is never 

 doubted. Again, the heart of Appendicularia has more the 

 appearance of a reduced than of a primitive organ. Finally, 

 the so-called vertebration of the tail of Appendicularia (Lan- 

 gerhans, 26, and Lankester, 27) presents nothing that is not 

 readily intelligible on the view that Appendicularia has 

 acquired its pelagic life secondarily, when one would expect to 

 find the muscular and nervous system of the tail, which has 

 now become a permanent organ, in a higher state of elabora- 

 tion than in the tails of the true larvse which only last for a 

 few hours. 



It results from these considerations, therefore, that Appen- 

 dicularia is probably not a primitive form, but represents a 

 larval Ascidian which has secondarily become pelagic and 

 progenetic. 



8. The Longitudinal Muscles of Ciona. 



The longitudinal muscles of Ciona have apparently the same 

 primary characteristics as those of Clavelina, as described by 

 Seeliger, but they appear at a much later stage. 



Seeliger says (loc. cit., p. 43), " Interessant ist das Auf- 

 treteu der Liiugsmuskulatur noch im freischwimmenden Lar- 



