ECHINODEEM LARV^. 479 



Mortensen's, and my own figures represent the same 

 organism. Speculation as to the adult form of this larva 

 is of little value ; but I would like to record my impression 

 that it will eventually be found to be Ophiopholis 

 aculeata. 



The larva of Ophiothria fragilis, figs. 32 to 36, PL V, 

 is easily recognised by its exceptionally long, straight 

 postero-lateral arms, which are banded at intervals with 

 black pigment, and persist until the metamorphosis of the 

 larva is complete. Owing to the comparative thickness 

 and consequent opacity of its body, and to the quantity 

 of pigment developed in the tissues, the later stages of 

 the development of this larva, and the appearance of the 

 primary elements of the skeleton of the adult, were much 

 less easily observed than those of ophiopluteus mancus. 

 In fig. 33, which is a ventral view of a young pluteus of 

 this species, the anterior and posterior coelomic vesicles 

 have taken up their respective positions, and the left 

 anterior one is about to divide into anterior and posterior 

 portions. In fig. 34, also a ventral view, this has taken 

 place, and the posterior portion of the left anterior vesicle 

 is the hydrocoel. On the right, the posterior vesicle, 

 though defined, is still continuous with the anterior one. 

 In fig. 35, which should be compared with fig. 29, PI. IV, 

 a stage is represented in which all the primary plates of 

 the skeleton are present and occupy their respective 

 relative positions in the now clearly defined disc of the 

 post-larva. The latter, freed from all traces of the 

 pluteus, and having five distinct arms provided with sub- 

 terminal claws (fig. 36), was abundant in the plankton 

 in late July and early August in the years 1907 and 

 1908. In the same months of 1909 this larva was only 

 sparingly represented. It was not observed by me in 

 July in the three succeeding years, but re-appeared in 



