STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 209 



canal system, from the dermal pores to the prosopyles. If we 

 consider the manner in which the inhalant canal system of the 

 more highly developed Heterocoela has been derived from the 

 Sycetta condition, by the closing in of a space which originally 

 lay altogether outside the sponge, it is obvious that Schulze^s 

 view as to the extent of the ectoderm must be correct. 



I also believe that Schulze was perfectly correct in his opinion 

 (7) as to the structure of the ectoderm. In the caseof Grantia 

 labyrinthica I stated that the ectoderm resembles exactly 

 what Schulze has described in Sycon raphanus, " consisting 

 of a single layer of flat, polygonal epithelial cells lining the 

 dermal surface of the sponge and the inhalant canal system. 

 These cells are most readily distinguished around the inhalant 

 canals, where they are less obscured by spicules and other 

 mesodermal structures than on the dermal surface. The 

 nucleus is surrounded by the very characteristic granules de- 

 scribed by Schulze in Sycandra. In my preparations I have 

 only after some trouble succeeded in making out the boundary 

 lines between the individual cells, and Schulze himself observes 

 that it is remarkable that the boundaries of these cells — some- 

 times so distinct — are not always clearly visible. Nevertheless 

 I have been able to determine the shape of the cells pretty 

 accurately, and found them to agree precisely with Schulze's 

 drawings " (9). 



Since the above was written I have also described and figured 

 very carefully the ectodermal epithelium of a Homoccele sponge, 

 Leucosolenia Wilsoni (1). '*^It consists of thin, flattened, 

 plate-like cells, polygonal in outline, and each with a swelling 

 in the centre where the nucleus is situate. The cell itself 

 averages about 0-0136 mm. in diameter, and the nucleus, 

 which is very distinctly outlined and more or less spherical 

 (or perhaps somewhat flattened in the same manner as the 

 cell), has a diameter of about 0*0034 mm. Within the nucleus 

 appear a few small, deeply staining granules. Around the 

 nucleus the protoplasm is highly granular, exactly as described 

 by Schulze, while towards the periphery of the cell it becomes 

 gradually hyaline. Adjacent cells are in contact at the edges. 



