STUDIES ON THE PROTOCHORDATA. 335 



5. First pair of gill-slits. 6 and 7. Second and third pairs of 

 gill-slits. In Ciona, as shown above, the first gill-slit of each 

 side is composed of the primary stigmata i, ii, iii, and iv. 

 These stigmata serve the young Ciona for respiration for a con- 

 siderable period, during which it is feeding itself up for further 

 growth. 



In Amphioxus, as I have previously shown (loc. cit., p. 225), 

 the first pair of gill-slits consists of the first primary gill-slit 

 proper and the club-shaped gland. This pair of gill-slits, of 

 which apparently only one member functions as a true gill-slit, 

 alone serves the young larva of Amphioxus for respiration during 

 a similar period, namely, the first period of independent feeding, 

 when there is not much progressive development, but the larva 

 is preparing itself for the future effort of growth. In Amphi- 

 oxus, towards the end of the larval development, the club- 

 shaped gland and the first gill-slit proper, which played such 

 an important role in the early stages, simultaneously close, 

 atrophy, and disappear (33). In both the Ascidians and 

 Amphioxus, therefore, the first pair of gill-slits has peculiarities 

 which distinguish it from all the succeeding gill-slits. 



From these considerations, and on account of its topographi- 

 cal relations, I conclude that the first pair of gill-slits of 

 Ciona, consisting of the first four primary stigmata 

 on each side, is homologous with the first pair of 

 gill-slits of Amphioxus. 



The second and third pairs of gill-clefts, of course, follow 

 suit. 



The above conclusion is in direct opposition to that 

 arrived at by van Beneden and Julin (5 and 7). In conse- 

 quence of their theory of the prechordal vesicle they denied 

 that any homology existed between the stigmata of the 

 Ascidians and the gill-slits of Amphioxus. They say, for in- 

 stance (5, p. 360), ''Nous ne pouvons done admettre I'homo- 

 logie entre les stigmates des Ascidiens et les fentes branchiales 

 de FAmphioxus j^' and further, ''Les Ascidiens comme les 

 Appendiculaires sont des Chordes pourvus d'une seule et 

 unique paire de fentes branchiales." 



