Quarterly journal of microscopical science . hiurids and Asterids. Ludwigs study of the development of the Ophiurid skeleton^has thrown a curious light on the observations of some of his ^ , those of Sir Wyv. Thomson, Dr. Carpenter, and M. Sars. 2 Gottes attempt to disprove the annular origin of the centrodorsal isnot a satisfactory one. Even supposing that he is right, and that thecentrodorsal does originate out of five primitively separate elements, itcannot, on the views advocated above, represent the primitively simpledorsocentral of the Echinozoa. ^ Dyreriget, * En Haand. og Laerebog
Quarterly journal of microscopical science . hiurids and Asterids. Ludwigs study of the development of the Ophiurid skeleton^has thrown a curious light on the observations of some of his ^ , those of Sir Wyv. Thomson, Dr. Carpenter, and M. Sars. 2 Gottes attempt to disprove the annular origin of the centrodorsal isnot a satisfactory one. Even supposing that he is right, and that thecentrodorsal does originate out of five primitively separate elements, itcannot, on the views advocated above, represent the primitively simpledorsocentral of the Echinozoa. ^ Dyreriget, * En Haand. og Laerebog til brug ved hojere Laerea^stal-ter, Kj0benhavn, 1882, p. 597. ^ Zeitschr. f. wiss. Zool., Bd. xxxvii, p. 60. Zur Entwickelungsgeschichte des Ophiureuskeleltes, Zeitsch. Zool., Ed. xxxvi, pp. lSl-200, Taf. x, xi. 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), afterLudwig. The numbering of the apical plates is the same in this andsubsequent figures as in the figures to my first paper (this Journal,vol. xviii). 1. Dorsoceutral. 2. Primary radials. 3. Basals orgenitals. 4. Radials. 5. Orals. T. Terminalia. ad.^. Secondadambulacral plates, Water-pore. (fig. I, t; fig. iVj 4). As the rays grow these terminal platesare carried outwards from the disc by the development of newplates between them and it. In the young Asterid they are sepa-rated from the dorsoceutral by the ring of rudimentary genitalplates (fig. IV, 3), and eventually by other intermediate plates{mi). But in the early Ophiurids the plates directly betweenthe terminalia and the dorsocentral are radial in position (, 4), and not interradial^ as the genitals are in the , while the two rows of plates around the dorsocentral ofthe young Asterid are alte
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