48 I. IJIMA : HEXACTINELLIDA. I. 



The oscularia are small or medium-sized but mostly stout 

 spicules with a variable number of rays. Nevertheless, many 

 Eupledella species have each a certain characteristic form 

 predominating amongst them, so that they are of considerable 

 importance in the systematic of the genus. They lie, not in a 

 single layer, but superposed in several layers. The more deep- 

 ly situated and also those in the periphery of the ring are the 

 larger ; and especially the thetactin or diactin forms in these 

 situations lead over the oscularia into the parenchymalia on the 

 one hand, while on the other hand certain pentaetin- forms 

 mediate their transition into the gastralia. It is then not without 

 justification that F. E. Schulze has referred them at one time 

 to the parenchymalia {E. aspergUlum, oiveni ; '87) and at another 

 to the gastralia [E. regalis ; 19', p. 20). 



The oscularia begin to develop some time after the first 

 breaking through of the parietal oscula ; hence, they may be 

 entirely absent or only scantily developed in quite young speci- 

 mens or in those oscula which have but comparatively recently 

 originated at the growing upper end of the body. This circum- 

 stance may at least partially account for the fact that in several 

 known species of the genus the oscularia have remaine 1 undis- 

 covered. I can not explain their non-occurrence around the 

 sieve-plate meshes, unless it be that the rigidity, which they 

 undoubtedly give to the parts occupied by them, is here sufTici- 

 ently provided by the compactly arranged parenchymalia and 

 gastralia so as to make their presence dispensable. 



The category of spicules or spicular rays, collectively called 

 the prostalia, is in all cases either intimately associated geneti- 

 cally with the parenchymalia or may even be the protruded 

 parts of the parenchymalia themselves. In Eupleetella, three sorts 



