254 I. IJIMA : HEXACTINELLIDA. I. 



bly quite free of other spicules which would have remained 

 sticking to them, had a sieve-plate been mechanically torn off; 

 and 2) that their shagreen-like surface — a feature which is also 

 shown by all the prostalia on the cuff-margin as well as on the 

 lateral surface — has apparently arisen in relation to the free 

 exposure of the parts thus characterized. 



At the same time there is no denying the fact that the corona 

 had been ontogenetically derived from a sieve-plate. I assume 

 that in an early developmental stage this comes into actual 

 formation, if only to a partial extent, but that its component 

 spicules are however soon loosened and lost as a normal process, 

 leaving behind permanently only those that are deeply rooted in 

 the lateral wall, viz., the coronal spicules. Genetically, therefore, 

 the corona and the sieve-plate are to be considered as very 

 nearly related structures, strikingly different though they appear 

 to be. In this light the large terminal osculum should plainly 

 and exactly correspond to the area which in certain other Hexac- 

 tiuellids is covered by a sieve-plate. 



Since H. komeyamal and R. phœnix show a far-reaching 

 similarity in other points of their organization, I am certainly 

 not disposed to find in the corona of the former anything of more 

 than specific value. 



The cuff is tolerably well developed in a continuous ring 

 (PI. IX, fig. 3). Breadth up to 9 mm., as measured on the 

 upper side. It is expanded outwardly and slightly inclined up- 

 wards. While its superior surface is comparatively flat and well 

 marked olf from the gastral surface by the angular oscular edge, 

 the inferior surface slopes down to merge insensibly in the 



