NOTES ON ECHINODERM MORPHOLOGY. 



379 



predecessors respecting the abactinal system. In the Ophiurids, 

 as in the Asterids, there is a dorsoceutral plate (figs, i, iv), 

 around which are the five terminal plates of the future arms 



Fig. I. — Apical system of a young Opliiurid {Amphiura squamata), after 

 Ludwig. The numbering of the apical plates is the same in this and 

 subsequent figures as in the figures to my first paper (this Journal, 

 vol. xviii). 1. Dorsoceutral. 2. Primary radials. 3. Basals or 

 genitals. 4. Radials. 5. Orals. T. Terminalia. ad.^. Second 

 adambulacral plates, w.-p. Water-pore. 



(fig. I, t; fig. iVj 4). As the rays grow these terminal plates 

 are carried outwards from the disc by the development of new 

 plates between them and it. In the young Asterid they are sepa- 

 rated from the dorsoceutral by the ring of rudimentary genital 

 plates (fig. IV, 3), and eventually by other intermediate plates 

 {mi). But in the early Ophiurids the plates directly between 

 the terminalia and the dorsocentral are radial in position (fig. 

 I, 4), and not interradial^ as the genitals are in the Starfish. 

 Hence, while the two rows of plates around the dorsocentral of 

 the young Asterid are alternate in position (fig. iv), those sur- 

 rounding the corresponding plate in the young Ophiurid are 

 both radially situated, so that there are two plates in the direction 

 of each ray, an inner and an outer one (fig. i, 4, t).^ 



^ The presence of the orals (5) in the stage represented in Fig. I some- 

 what obscures this arrangement. It is better shown in the earlier stage 

 represented in Ludgwig's fig. 17, where the only oral visible is that which 

 ultimately bears the madreporite. 



