214 ARTHUR DENDY. 



extremely conspicuous in sections of osmic acid specimens 

 (fig. 23). 



Sometimes in sections of ordinary spirit-preserved material, 

 cut by the paraffin method, the flattened cells of the ecto- 

 dermal epithelium appear separated from one another by 

 considerable intervals, as is shown in fig. 29, representing a 

 portion of the epithelial lining of the inhalant canal system of 

 Grautessa intusarticulata, and in fig. 59, representing 

 epithelium from a corresponding situation in Sycon K-amsayi. 

 This appearance is evidently due to contraction of the cells, 

 which may still remain connected in places by strands of 

 protoplasm which stretch across from one to the other. Both 

 ectodermal and endodermal epithelial cells appear to be very 

 subject to such contraction, and it appears to me very possibly 

 to indicate a normal contractility in life, such as is described 

 by Metschnikofi" (29). 



It has been recently maintained by Bidder (23) and Minchin 

 (26) that the prosopyles which pierce the walls of the flagel- 

 lated chambers are formed each by the perforation of a single 

 nucleated cell ; but, while Bidder attributes to these cells an 

 endodermal origin, Minchin regards them as derivatives 

 of the ectoderm. For my own part I am disposed to regard 

 the prosopyles as inter-cellular and not intra-cellular in 

 nature, and Schulze^s admirable drawings of the anatomy of 

 Sycon raphanus (7) point to the same conclusion. My own 

 drawings of the anatomy of Grantia labyrinthica show 

 exactly the same condition, and in the very numerous sections 

 of calcareous sponges which I have examined, I have met with 

 no evidence to cause me to doubt the correctness of the view 

 that the prosopyles are simply gaps between cells. Bidder, 

 indeed (21), interprets the remarkable groups of yellow granules 

 which I described in Leucosolenia cavata (1) as perforated 

 prosopyle cells. I do not deny the possibility of this view, but 

 I do not think that the evidence is sufficient to justify it, 

 and, in any case, the occurrence of such structures (whatever 

 they may be) is in my experience very rare even in the 

 Homocoela, and altogether unknown in the Heterocoela, of 



