TII1<; DEVELOPMENT OF OPUIOTHIUX FUAGILIS. 587 



developmeut. In one specimen (fig. 53), it had actually 

 assumed a five-lobed form, presenting a resemblance to the 

 shape assumed by its left fellow at an earlier period of 

 development. Its normal form may be regarded as that 

 shown in fig. 47, viz. a small vesicle with a comparatively 

 thin wall. But quite often it is large, thin-walled, and 

 conspicuous, sometimes, indeed, forming qnite a projection on 

 the external surface of the larva. Sometimes a projection of 

 its inner wall is noticeable similar to that which gives it a 

 crescentic form in the larva of Aster ina gibbosa. The 

 right anterior ccelom has disappeared with the alteration in 

 form of the oesophagus whilst the left anterior coelom forms, 

 as it does in both Aster in a gibbosa and Echinus 

 esculentus, the axial sinus or "'ampulla'^ of the stone- 

 canal. 



As is well shown in fig. 47, the madreporic pore has been 

 brought close to the right hydrocoele, although it was at first 

 widely separated from it. This is due to the rotation of the 

 madreporic pore from its original position on the left side 

 over the oesophagus and so on to the right side. Thus is 

 eventually brought about a juxtaposition of organs which in 

 the larva) of Asterina gibbosa and Echinus esculentus 

 exists from the first. The position of the right hydrocccle in 

 these two species close to the mid-dorsal line of the larva, has 

 led both Goto (11) and Masterman (20) to combat my view of 

 its homology. But Ophiothrix fragilis is quite decisive 

 on the point, and a little consideration will show why the 

 Ophiurid in this respect retains a much more primitive 

 arrangement than the other two forms. In it the larval 

 mouth is gradually transformed into the adult mouth, and 

 instead of the mouth moving to the left the stomach is 

 displaced towards the right (fig. 47, st.) ; in the Asterid and 

 Echinid, on the other hand, the permanent mouth is formed 

 on the left, whilst the stomach retains its original position. 

 The right hydrocoele in these two groups is orientated with 

 respect to the eventual position of the permanent mouth, and 

 " right " with respect to a mouth which is going to be 



