146 ARTHUR wn,T,EY. 



and tliouo-ht it ])robable that the size of the stig-mata was an 

 indication of their ordei' of formation. It may or it may not 

 be so in this particular case, bnt nothing can be affirmed 

 without special investigation. 



In his valuable "Etude de la branchie chez Ascidiella 

 scabroides/' M. de Selys Longchamps^ criticises my state- 

 ments concerning the independent perforation of protostig- 

 mata V and VI in Ciona. He says (p. 121) : 



"En fait, rien n'est plus difficile que d'etablir qu'un 

 stigmate est ne par perforation ; je me demande meme sur 

 quel fait positif Willey se base, pour attribuer une origine 

 independante aux protostigmates V et VI chez Ciona." 



To this I can reply that, apart from the general conviction 

 resulting from the comparison of numerous specimens, the 

 positive observation which demonstrated to me the fact of the 

 independent perforation of protostigma V is illustrated in my 

 PI. XXX, fig. 14 (op. cit.). Here it will be seen that the 

 ventral ends of the protostigmata I and IV are inclined 

 respectively towards their derivatives II and III, while V 

 appears as a minute perforation apart, and is later followed 

 by VI without any indication of abstriction. 



Moreover Molgula manhattensis will afford collateral 

 evidence in corroboration of my former account of Ciona. 



5. Adolescent Period. — If it be thought useful to 

 recognise different periods of development it is necessary to 

 be precise. Fortunately this is comparatively easy with the 

 simple Ascidians, which exhibit five well-marked periods, 

 namely: — 1. Embryonic. 2. Larval. 3. Nepionic. 4. Ado- 

 lescent. 5. Adult. 



The nepionic period obviously commences with the fixation 

 of the larva, but there is a slight difficulty as to its termina- 

 tion, due to the fact that there is no such abrupt contrast 

 between the nepionic and adolescent periods as there is 

 between the former and the larval period. It is a question 

 whether it should be regarded as ending with the establish- 

 ment of the six protostigmata or with the subdivision of these 



' 'Archives de Biologip,' xvi, ]S*)9, pp. 117 — 3 71. 



