THE DEVELOPMENT OF OPHIOTHHIX FliAGlLIS. 577 



"ampulla" of the stone-canal, the equivalent of the axial sinus 

 in Aster in a gibbosa. About the fourth day the post-oral^ 

 arms make their appearance and the ventral side of the larva 

 becomes mai-kedly concave. The new arms are supported by 

 outgrowths from the primary skeletal rods, which arise at the 

 same point as those which support the antero-lateral arms. 

 The left coelomic sac now grows out into a posterior tongue of 

 cells at first solid (fig. 32, l.jhc), which soon hollows out to 

 form a vesicle (fig. 33). This vesicle becomes separated from 

 the left coelomic sac as the left posterior coelom, and moves 

 back so as to be at the side of the stomach (fig. 8, l.i^.c), whilst 

 the original sac now termed the left anterior coelom (fig. 8, 

 l.a.c.) remains at the side of the oesophagus. About the fifth 

 day the right coelomic sac undergoes a similar series of changes. 

 The solid posterior outgrowth is shown in fig. 34, whilst in 

 fig. 8 is represented a specimen in which this outgrowth {r.'p.c.) 

 having become a hollow vesicle is in the act of separating 

 from the original sac, now termed the right anterior coelom 

 {r.a.c), which remains at the side of the oesophagus. In fig. 9, 

 a slightly older specimen is shown in which the division is 

 complete on both sides. Thus in Ophiothrix fragilis as 

 in Asterina gibbosa and in Echinus esculentus the 

 transverse division of the coelom of the left side precedes that 

 of the right, thus foreshadowing that predominance of the 

 left side which plays such an important part in the meta- 

 morphosis. 



The larva now continues to grow in size without under- 

 going any important changes for a day or two, but at a 

 period varying from seven to ten days from fertilisation, 

 according to the amount of food-supply present in the water, 

 the last pair of arms, the postero- dorsal - (figs. 10 and ll,p.fZ.) 



' Owinstoan unfortunate misunderstanding of Morleusen's paper (22) the 

 arms of the larva of Echinus esculentus are wrongly named in my paper 

 on the subject (19). What 1 have called in that paper postero-dorsal arms 

 arc really post-oral and vice- vers a. 



'•= These arms correspond to those arms of the larva of Echinus 

 esculentus, wrongly named post-oral by me in my paper on the subject. 



