THE DEVELOPMENT OF OPHlOTHlilX FRAGILIS. 565 



off from the apex of the archenteron. A large celkilar plug- 

 projects from the apex of the coelomic vesicle and extends 

 into the cavity of the archenteron. From this circumstance 

 Grave draws the somewhat rash conclusion that the ai'chen- 

 teron has been formed by the hollowing out of the inner mass 

 of a planula-like larva. In the next stage^ which is only a 

 few hours older, a remarkable state of affairs has come about. 

 The first vesicle is completely constricted from the archenteron 

 and divided into right and left halves. A second vesicle, 

 however, has been budded from the archenteron and is still in 

 open communication with it. This vesicle is already partially 

 divided into two by a constriction, and its anterior portion 

 shows the five lobes which proclaim it to be the hydrocoele, 

 whilst its posterior portion gives rise to the hypogastric 

 coelom, i.e. the left posterior coelom. This state of affairs 

 is widely different from that which has been described in the 

 case of any other Echinoderm. The very early appearance 

 of a lobed hydrocoele shows that the first stages of develop- 

 ment have been very much hurried over, and hence we must 

 regard differences from the normal type of Echinoderm 

 development as secondary modifications, not as indications of 

 a primary state of affairs. In the next stage the ectodermal 

 stomodajum has met the oesophagus. It passes under the 

 arch of the hydrocoele, so that this forms a bridge half-way 

 around it. The stone-canal is formed as a secondary connec- 

 tion between the left anterior coelom and the hydrocoele. A 

 so-called epigastric coelom — corresponding to the right pos- 

 terior coelom of the free-swimming pluteus — appears in this 

 stage, and after some discussion of the point. Grave concludes 

 that it must have been formed from the right anterior coelom. 

 In the next stage the cilia are restricted to four transverse 

 rings, which produce an appearance of metameric segmenta- 

 tion and give rise to the " worm-like " appearance of the 

 larva. The posterior end of the larva is broadened, and 

 contains practically all the organs of the Ophiurid; the 

 anterior portion contains mesenchyme and forms a large 

 pra)-oral lobe. The hydrocoele has rotated to such an extent 



