152 I. IJIMA : HEXACTINELLIDA. I. 



stains. It will then require no further words to explain that I 

 regard the trabeculte as consisting of the fused cytoplasm of the 

 cells represented by the above free auclei. Here I leave this 

 point, to resume it soon again. 



The question whether the trabeculse have a pinacocyte covering 

 or not, I am fully aware, is a delicate one. F. E. Schulze has 

 always assumed its presence in the forms studied by him, an 

 assumption which seems quite justifiable from a theoretical point 

 of view. However, as before indicated, I have been led to the 

 contrary opinion, though at first I felt much diffidence in coming 

 to this conclusion. Mention has already been made (p. 33) of 

 my failure to demonstrate cell-outlines on the trabecular surface. 

 The history of our knowledge of the pavement epithelium in the 

 Spongida teaches us caution in drawing conclusions from that 

 negative lesult ; but in the pi-esent case, I am quite at loss to 

 believe that my methods were in any way at fault. Besides, 

 there is another circumstance, which, simple as it is, seems to 

 me to deserve due consideration. It is the excessive thinness in 

 which the trabeculfe, whether filamentous or membranous, so 

 often present themselves in all parts of the sponge-body (see 

 PL V, fig. 43, Ir.; also PI. VIII, fig. 30). The thinness is such 

 that barely enough room is given for the protoplasmic granules 

 to arrange themselves in a single row or layer, as tlie case may 

 be, and this seems to be scarcely compatible with the assumption 

 of the plurality of differentiated tissues or layers. To all appear- 

 ance, the thinner trabecuhe are nothing more than simple thi'eads 

 or films of the protoplasm. In the absence of indications to the 

 contrary, I see no ground for hesitating to ascribe the same 



