28 



that it is almost impossible to discern their shape. Gradually this rotation 

 ceases completely. Two days old they have the shape of young Plutei, 

 with a red pigment spot at the end of the postoral arms and scattered 

 pigment cells in the body. The suboral cavity apparently is not so large 

 as in Diadema.^) (PI. V, Fig. 6). 



The skeleton (Figs. 3—4). The body-skeleton forms a basket structure 

 as in Diadema, but differs from the latter in being generally conspicuously 

 thorny. Sometimes the lower part may be fenestrated, which means. 



Fig. 3. 



Skeleton of the larva of Astropyga pulvinala, 

 5 days old. =»7i. 

 3. A. dorsal view; B. end view. 4. A— G. side view, 

 showing individual variations. Letters as in flg. 1. 



evidently, only an unusual development of the thorns. The transverse 

 rods connecting the recurrent rods ( — not the same as the dorsal trans- 

 verse rod of the later stage, which is a process from the base of the postero- 



Echinoderms reared by me "in swimming rotate around their longitudinal axis, always 

 turning to the left". Gemmill (On the development and certain points in the adult struct- 

 ure of the Starfish Asterias rubens. Philos. Transact. B. 205. 1914. p. 238) finds this state- 

 ment difficult to understand, since he has found the rotation to be dextral. I am myself 

 at a loss to explain this statement. In my notes from the stay in Plymouth I have repeatedly 

 stated expressly that the rotation is always to the right. The error is not even due to a lapsus 

 memorise; I have been too impressed with the observation of this constant dextral rotation. 

 In fact, I have no excusion to offer for this erroneous statement. I may take the opportvmity 

 here to say that I have not seen any of the numerous Echinoderms, which I have reared, to 

 make an exception to this rule of dextral rotation; only now and then a single embryo ma> 

 be seen to turn to the left. But I have observed that only very rarely. 



1) On the figure of this larva, which was drawn from life, the suboral cavity was not 

 indicated. The line representing the lower limit of this cavity was added to the figure after 

 preserved specimens; possibly it is larger in the living specimens and really of the same 

 extent as in Diadema. 



