ANATOA/Y AND DEVELOPMENT. 233 



Ciona, have a somewhat different mode of origin. The 

 two first-formed stigmata (= I. and IV. in Ciona) appear 

 simultaneously as in Ciona. Then after growing to a cer- 

 tain size, they curve round at their ventral ends, not in 

 opposite directions so as to meet each other as they do in 

 Ciona, but in the same direction (Fig. no). The recurved 

 ends then become constricted off from the parent stig- 

 mata. Later on, a fifth gill-opening arises behind the 

 first four stigmata by independent perforation, and after 



W 



Fig. no. — Diagram illustrating the mode of origin of the six primary bran- 

 chial stigmata of Molgula manhattensis. The numbers are placed at the ventral 

 ends of the slits. The figure is a combination of several hitherto unpublished 

 drawings of different stages in the development. /, ///, and F arose by separate 

 perforation. 



attaining a certain size, it, in its turn, curves round at its 

 ventral end, and eventually the sixth stigmatic opening is 

 constricted off from the fifth. 



Since the first six primary stigmata have such different 

 origins in two different species, it is obvious that in 

 attempting to make a comparison with Amphioxus we can 

 only use the two first-formed stigmata, because they agree 

 in the above-mentioned species, and in many others in 



