1892.] 



Development of Ciona intestinalis, §c. 



515 



4 



Fig. 1. — A young Ciona, shortly after fixation. From the right side. Drawn with 



cam. luc., Zeiss 3 C. 



!N".B. — The atrial sperture is merely the external aperture of the first gill-slit. 



'Explanation of Letters. — t., remains of tail; 02., oesophagus ; a., atrial aperture of 

 this side; 1st g. s., first gill-slit (in Ciona, double from the beginning) ; e., eye; 

 m., mouth ; ed , endostyle ; p. c, proboscis cavity; d., adhering disc ; p., pericardium ; 

 %., exit of intestine from stomach; s., stomach. 



Phallusie," 'Arch, de Biologie,' vol. 5, 1884) commenced with, indivi- 

 duals possessing four on each side. In all cases they were supposed 

 to represent gill-slits which had developed by independent perfora- 

 tions. 



Van Beneden and Julin, judging from the sizes of the slits, came 

 to the conclusion that they formed in a very irregular manner, the 

 first slit formed being the fourth of the series, and so on. In other 

 words, the true first gill-slit of the Ascidians has been, up to the 

 present, unknown: and it is owing to the ignorance which has 

 hitherto prevailed with regard to this slit that the relations between 

 the Ascidians and Amphioxus have been so little understood. Thus, 



