820 ARTHUR WILLEY. 



That this is a correct statement of the situation will become 

 evident to the reader of the following pages ; and the reason 

 for it is to be found in the fact that most zoologists who have 

 worked at this part of the development of Ascidians have not 

 had a sufficient number of different stages before them. 



Several observers, including P. J. van Beneden (4), Krohn 

 (23), Kowalevsky (20), and KupfFer (24), have seen young 

 Ascidians with two gill-slits on each side; but they have 

 neither described the exact origin of these, nor have they, with 

 the exception of Krohn, described the formation of new slits. 

 JEd. van Beneden and Julin (6), in their account of the post- 

 embryonic development of Phallusia scabroides, com- 

 menced with the stage at which four gill-slits were present on 

 each side. The transition from the stage with two pairs of 

 branchial apertures to that with four forms precisely what 

 Kowalevsky would call a " Hauptmoment" in the development, 

 and has been overlooked by all previous observers. 



In their youngest specimen of Phallusia scabroides van 

 Beneden and Julin found that the four slits were of unequal 

 size, and thought it probable that the sizes of the slits indi- 

 cated their order of formation as follows : — iii, i, iv, ii. In 

 the next stage which they examined six stigmata were present 

 on each side, and they constructed the following table to show 

 the relation which they supposed to exist between the age (as 

 determined by the size) and the order of position of the respec- 

 tive slits : 



Ordre de Position. 



Age relatif. 



Premier stigmate forme. 

 Second „ „ 



Troisieme „ „ 



Quatrieme „ „ 



Cinquieme „ „ 



Sixieme „ „ 



Quatrieme de la serie. 

 Premier „ „ 



Cinquieme „ „ 

 Second „ „ 



Troisieme „ „ 

 Sixieme „ ,, 



In their ' Recherches' (7) the same authors recur to this 

 assumed irregularity in the formation of the primary stig- 

 mata, and lay great stress upon it as showing that the gill- 

 slits of Ascidians are not in any sense true metameric organs. 



