138 I. IJIMA : HEXACTINELLIDA. I. 



and sparsely granular. As known through F. E. Schulze, the 

 nucleus is remarkably poor in chromatin, on which account, 

 it, unlike all the nuclei of other tissues, does not surpass the 

 surrounding protoplasm in staining capacity. Nor does it con- 

 tain a body which might be called the nucleolus. Hence, in 

 certain Hexactinellid species or in a certain state of preservation, 

 the nuclei of the choanocytes can be demonstrated only with 

 difficulty. In the surface view of the chamber-wall stained with 

 acid-fuchsin (fig. 37), a clear ring is observable around the nu- 

 cleus at a certain focus of the microscope ; this is apparently due 

 to the refrangibility of the nuclear substance. Also a highly 

 refractive spot is visible in a central position in each nucleus ; 

 this is due to the origin of the flagellum, which arises directly 

 from the inner (distal) surface of the nucleus, and should not 

 be mistaken for a nucleolus. 



To F. E. Schulze belongs the credit of having first demon- 

 strated the Hexactinellidan flagellum and collar in Schaudinnia 

 arctica. The former structure had been long known to me from 

 Eupleciella marshalU and Acanthascus cactus, in both of which 

 it is fairly constantly preserved in preparations fixed with cor- 

 rosive sublimate. As the total length of the flagellum I may 

 put down 17-19 p-. In the profile view it appears as a fine line, 

 very faintly stained by acid-fuchsin (figs. 40-42). In the surface 

 view of the chamber-wall (fig. 37), it appears in optical section 

 as a glittering dot, which, by varying the focus of the micros- 

 cope, may be made to move continuously away from, or toward, 

 the central refractive spot of each nucleus, as the case may be. 



As to the collar, my opinion was very uncertain for a long 



