208 AETHUR DENPy. 



reached the radially symmetrical condition, but to have 

 remained in the scattered one found amongst the reticulate 

 Homoccela. 



IV. — The Histology of the Heteroccela Calcarea. 



All my observations upon the histology of the Heterocoela 

 Calcarea have been made upon spirit-preserved specimens, 

 and I cannot therefore claim for them the same value as 

 attaches to the recent observations of Bidder (21 — 24) and 

 Minchin (25, 26), who were able to study the sponges in the 

 living condition. Nevertheless even a cursory examination of 

 my figures 23 to 64 will, I hope, show that there is a good 

 deal of histological detail to be made out even in spirit- 

 preserved specimens of calcareous sponges. 



With regard to the classification of the tissues, I still main- 

 tain the opinion which I expressed in my memoir on the 

 anatomy of Grantia labyrinthica (9) — that is to say, I still 

 follow Schulze (27) in considering that the ectoderm of 

 the larval sponge (at any rate in the Calcarea) furnishes not 

 only the epithelium of the dermal surface, but also that of the 

 entire inhalant canal system ; that the endoderm lines the 

 remainder of the canal system (including, of course, the 

 flagellated chambers) from the prosopyles to the osculum, 

 and that the mesoderm furnishes all the remainder of the 

 sponge body. 



As to the homology of the three layers in sponges with the 

 similarly named layers in higher animals, I do not presume to 

 offer an opinion. It is sufficient for our present purposes 

 that three layers exist in sponges; and as the names ectoderm, 

 mesoderm, and endoderm have come into general use for these 

 layers, and serve admirably to express their relations one to the 

 other, I naturally adopt them in this place. 



The Ectoderm. 

 This forms a single layer of epithelial cells which lines the 

 external surface of the sponge, and also the whole inhalant 



