24 ARTHUR DENDT. 



possibly be due to shrinkage. The apices of the cells are still 

 further apart^ and in longitudinal sections (fig. 33) are seen to 

 be connected by a fine, sharp line running from one to the 

 other. This line is Sollas's membrane seen in section, no 

 longer supported on the tops of the collars, which have been 

 retracted, drawing the membrane after them close down into 

 the apices of the cells. So Sollas^s membrane remains visible 

 even when the collars of the cells are retracted, which indicates 

 that it is probably a more or less permanent structure, and no 

 mere temporary fusion of the margins of adjacent collars. 



Owing to the great transparency of the gelatinous meso- 

 dermal ground-substance — which is a very characteristic feature 

 of calcareous sponges — it is not possible to determine, as in the 

 case ofStelospongos, exactly how far it extends between the 

 collared cells. Probably there is considerable individual varia- 

 tion in this respect, and it is a matter of little importance to 

 ascertain exactly how far each cell happens to be sunk in the 

 matrix. I cannot believe, however, that the actual collars are 

 ever embedded. 



I understand that Mr. Bidder has already pointed out that 

 Sollas's membrane occurs in a calcareous sponge, Leuconia 

 asp era, but I have neither been able to see his paper nor to 

 find out where it was published. 



The Mesoderm. 

 The constituents of the mesoderm may be divided into two 

 main classes, the cells and the cell-products. We will con- 

 sider the cell-products first ; they consist of the intercellular 

 ground-substance and the spicules, and as the spicules have 

 been already fully described we have only to deal with the 

 ground-substance. The ground-substance ' consists of the 

 usual transparent jelly, exhibiting no differentiation excepting 

 a slight concentration around the spicules, forming the so-called 

 spicule sheaths. These are very distinct, and are always seen 

 to be continuous with the surrounding ground -substance 

 when the spicules themselves have been dissolved out by the 

 1 Maltha of Ilaeckel (12). 



