134 HIROSm OHSHIMA 



an isolated hydrocoelo : (a) it may have been separated from 

 the end of the stone-canal, or (b) the posterior coelom may have 

 given rise to it under the influence of the amniotic invagination. 

 From the absence of posterior coelom, though one appeared 

 afterwards, he thinks the latter more probable. In one of 

 Eunnstrom's larvae of inverse situs (Case B) we see 

 another extraordinary feature in the right hydrocoele (23, 

 p. 9 ; 26, p. 428). The hydrocoele was three-lobed, and close 

 to it there were two curious structures. One was a round 

 closed vesicle, the origin of which the author could not ascertain. 

 The other was an ectodermal groove running nearly parallel 

 to the stone-canal and lined with very actively-moving cilia. 

 This groove at last became separated from the ectoderm, 

 and together with the above-stated closed vesicle, united with 

 the hydrocoele, remaining as a larger lobe of the latter. 

 Eunnstrom is of the opinion that in those pathological cases 

 a hydrocoele or a part of it can be formed both from posterior 

 coelom and ectoderm. 



(/) Amniotic Invagination. — This is formed some 

 days later than the appearance of the hydrocoele. It seems to 

 me highly probable that this structure is homologous with 

 the stomodaeal invagination of Holothurians. As early as 

 1906 Mac Bride (12, p. 615) pointed out that the larval 

 stomodaeum of Holothurians reminds one ' of the amniotic 

 cavity in the Echinopluteus '. This idea has since found another 

 support in the fact that in Cucumaria the stomodaeal 

 invagination is formed to the left of the mid-ventral line, as was 

 first discovered by Newth (20, p. 634, PI. i, fig. 6) and after- 

 wards confirmed by the WTiter (21, pp. 379, 384, PI. v, figs. 5 

 and 6). It is therefore quite improbable that the ancestral 

 Echinoid had a pair of amniotic invaginations. MacBride 

 (15, p. 343) never found in any single instance an amniotic 

 invagination formed where no hydrocoele existed, and con- 

 firmed his former view (14, pp. 240-1) that the undifferentiated 

 ectoderm can give rise to an amniotic invagination only under 

 the influence of the hydrocoele. Eunnstrom's view is 

 diametrically opposed to this. He has sho^ii us several 



