HEXACTINELLIDA 



199 



the Japanese investigator, Isao Ijima, 1 the dermal and gastral 

 membranes are but expansions of the trabeculae, and the 

 trabeculae themselves are entirely cellular, containing none of 

 the gelatinous basis met with in the dermal layer of all other 

 sponges. There is no surface layer of pinacocytes, the cells 

 forming the trabeculae being all of one type, namely, irregularly 

 branching cells, connected with one 

 another by their branches to form 

 a syncytium. In the trabeculae 

 are found scleroblasts and archaeo- 

 cytes. 



The chambers have a charac- 

 teristic shape : they are variously 

 described as " thimble -shaped," 

 " tubular," or " Syconate," and 

 they open by wide mouths into 

 the subgastral trabecular space. 

 Their walls have been named the 

 membrana reticularis from the fact 

 that, when preserved with only 

 ordinary precautions, they are seen 

 as a regular network of proto- 

 plasmic strands, with square meshes 

 and nuclei at the nodes. This 

 appearance recently found an ex- 

 planation when Schulze, for the first time, succeeded in preserv- 

 ing the collared cells of Hexactinellids. 2 Schulze was then 

 able to show that the choanocytes are not in contact with 

 one another at their bases, where the nuclei are situated, but 

 communicate with one another by stout protoplasmic strands. 

 The form of the choanocyte can be seen in Fig. 91. 



To Schulze's description of the chamber, Ijima has added the 

 important contributions that every mesh in the reticulum func- 

 tions as a chamber pore or prosopyle ; and that porocytes, such 

 as are found in Calcarea, are wanting. This structure of the 

 chamber -walls, the absence of gelatinous basis in the dermal 

 layer, and the slight degree of histological differentiation in 



Fig. 90. Portion of the body-wall of 

 Walteria sp., showing the thimble- 

 shaped flagellated chambers, above 

 which is seen the dermal mem- 

 brane. (After F. E. Schulze.) 



1 J. Coll. Japan, xv. 1901, pp. 128, 147, 190. 



2 Fauna Arctica (Roemer and Schaudinn), i. 

 Berlin, 1899, p. 98. 



1900, p. 84 ; and Sitzb. ATcad. 



