232 



THE ASCIDIANS. 



formation of tongue-bars, before the perforation of the 

 slits, has been described by Professor T. H. Morgan. 



From what has been said above, we conclude that the 

 first four pairs of primary branchial stigmata of Ciona 

 (and this probably applies equally to many species of 

 Phallusia) represent and are derivatives of one pair of 

 primitive, ancestral gill-slits. 



After a comparatively long interval, during which the 

 intermediate stigmata, II. and III., increase in length 

 transversely, two more stigmata, V. and VI., arise at inter- 

 vals, one after the other, by sepa- 

 rate perforations behind those 

 / /i^ .-«««*°T) already formed (Fig. 109). 



n ifT^ ^-tin'^ ^^ account of the independent 



origin of V. and VI., it might be 

 supposed that they would have 

 the morphological value of dis- 

 tinct gill-slits, and that we had 



yi -s^ before us three pairs of ancestral 



Fig. 109. -Primary branchial giU-slits represented by six pairs 

 stigmata of the right side of a Qf primary branchial stigmata. 



young Ciona. (After WiLLEY.) ^ " 



For this interpretation to hold 

 good, we should expect to find that in other forms in which 

 six primary branchial stigmata were produced, their origin 

 was either the same or reducible to the same type as that 

 of the branchial stigmata of Ciona. 



This, however, is not the case, since I have found 

 that in Molgiila manliattcnsis* a simple Ascidian which 

 occurs in great numbers at New Bedford, Mass., the six 

 primary stigmata, corresponding precisely to those in 



* My observations on the development of Molgida manhaUensis were 

 made at the Marine Biological Laboratory, at Woods HoU, Mass., in the 

 summer of 1S93. 



