E. MAKSHALLT. — DEYELOPJIEXT OF IIEXASTEES. 193 



teristic axial filaments in the principals (see p. 51). It may 

 therefore be said that a hexasier begins its development as a 

 hexaetin. The terminals are appendages which are later added to 

 the principals (see p. 56). 



It is highly nnsatisfactory that earlier stages than the one 

 jast mentioned — stages which might have shown the mode of the 

 first formation of hexaster principals and tliercfore of hexaetin 

 rays — have not been discovered. I should think it not unlikely 

 that these might be found to originate each as a separate sclerite, 

 as was so beautifully shown by Minchin ('98) to be the case 

 with the rays of triratliates and quadriradiates in the Calcarea.''' 



The hexradiate principals, during the entire period of develop- 

 ment of both the floricomc and the graphiocome, are imbedded 

 in a body of protoplasmic substance inclosing a crowded number 

 of nuclei. This nucleated substance may not improperly be called 

 the sclc7'oblast-7tiass, for reasons which I tliink are obvious. At 

 first, so long as the terminals are yet undeveloped or are very 

 sliort, the mass may be said to present a more or less octahed- 

 ral shape, with somewliat concave surfaces and with rounded 

 corners (figs. 29, 30, 32). In it the three axes of the principals 

 are disposed similarly to the axes in a crystal octahedron, the 

 outer ends of the principals coming u[) very close, but I think 

 normally not quite, to tlie surface at the six rounded corners. 

 The mass may otherwise be described as having its surface raised 

 into six, radially directed, hump-like protuberances by the six 

 principals contained within. Later, after the terminals have 



* I may here once more call attention to the facts, mentioned before, viz., that the 

 fii'st spicnies wliioh arise in tlie larv;e of Leucops(i(ms orthodocus and Vitrdlula fertile are 

 stanractimi, and that these are fonnod in tlio periphery of tlie inner cell-mass in a very early 

 larval stage which sliows as yet no indic.aiion irluileixr of the chambei'S, the flagellated cells still 

 farming a covering layer on the external surface. These facts may contain hints of great 

 significance as to the phylogeny of spicnles in the Hexaclinellida. 



