EXPLANATION OF PLATES 



All figures are drawn from living individuals; and form, proportions, and 

 axial relations are reproduced as exactly as possible, but they are otherwise 

 semi-diagrammatic in that details not essential to the purpose of the paper 

 are omitted. The ciliated band on the anterior end and arms of the pluteus is 

 drawn only in figures 5 A and B to indicate its normal relations and in figures 

 37 to 43, 47, 48, 57 to 72, 74 to 77, where its development is modified. In other 

 figures of plutei (figs. 6, 7, 23, 24, 29 to 36, 82 to 89), where its development shows 

 essentially the usual relation to that of the anterior end, it is omitted. The 

 mesenchyme cells are omitted in all cases, and the fenestration of the arm rods 

 and the posterior skeletal structure is merely diagrammatically indicated. In 

 figures of blastulae and gastrulae, all parts are drawn in double contour to indi- 

 cate the thickness of the body layers. In figures of stages intermediate between 

 gastrula and pluteus single contours are used, except for the thick-walled stom- 

 ach-intestine, and in figures of pluteus stages single contours are used throughout. 



Figures lettered A, B, C with the same number are, respectively, basal (anal) 

 lateral and anterior views of the same individual except in the case of figure 

 74 where A is a lateral and B an anterior view. 



PLATE 1 



EXPLANATION OF FIGURES 



Normal development 

 1 to 5 Elongated blastula to fully developed pluteus. 



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