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38o HIR0SH1 OHSIIIMA : 



Dipleurula. 



During the next stage of development the archenteron divides into 

 the hydro-enterocœle and the gut, and the former again into the hydro- 

 cœle and the enterocœle. The enterocœle again divides into the right 

 and left halves. This stage corresponds with the auricularia of other 

 holothurians so far as the arrangement of these vesicles is concerned, 

 and I may call the embryo " dipleurula " for the sake of convenience, 

 though the internal structure is never bilaterally symmetrical. The 

 stage is passed over at about the end of the second day of develop- 

 ment. 



The third and posteriormost part of the archenteron, the future 

 gut (Textfig. A, g'), is first pinched off from the rest, the hydro- 

 enterocœle. Part I above mentioned of the archenteron {hy') then goes 

 over from its original position on the left to the right, across the dorsal 

 side; while part 2 {eu') moves anteroventrai towards the left side, 

 from its original position on the dorsal side (Textfig. B). These two 

 parts are later constricted off from each other, resulting in the separa- 

 tion of the hydrocœle from the enterocœle (Textfig. C). 



From the posterodorsal corner of the hydrocœle is given out a 

 conical process, the rudiment of the primary stone-canal {sic'). This is 

 formed before the separation of the hydro-enterocœle, but its external 

 opening, the dorsal pore (Textfig. D, dp), appears some time after the 

 separation. The broad anterior margin of the hydrocœle is then 

 divided into several lobes. From the hydrocœle on its right ventral 

 edge is given out another lobe, which gradually grows on to the 

 ventral side, and brings about the horse-shoe shape of the future 

 ring-canal. 



The enterocœle which has been extending all along the left side 

 of the gut is then divided into two halves, one anteroventrai the other 

 posterodorsal. The one situated on the anteroventrai side corresponds 

 with the left enterocœle of other echinoderms (Textfig. D, len), while 



