STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 211 



offer some slight evidence of a similar contractility amongst 

 the Heterocoela. 



Taking into consideration all the above evidence, I think 

 we may regard it as an established fact that the ectoderm, 

 at any rate in all calcareous sponges, like that of Sycon 

 raphanus, normally consists of flattened plate-like poly- 

 gonal cells with centrally placed nuclei. I shall pre- 

 sently have to bring forward fresh evidence in support of 

 this view. 



A peculiar " flask-shaped" condition of the ectodermal cells 

 has, however, been described from time to time in various 

 sponges, and has recently received a good deal of attention 

 from Bidder and Minchin. 



This condition was first described by Metschnikoflf (28) in 

 an Olynthus form of Leucosolenia, and the peculiarity 

 appears to consist in the nucleus being suspended, as it were, 

 in an envelope of protoplasm from the under surface of the 

 plate-like cell. Bidder (21), in criticising my description of 

 the flattened ectoderm of the Homocoela, observes, " Although 

 this form occurs in the Homocoela, it is, in ray experience, 

 rare. The typical ectoderm (e.g. Ascetta clathrus) I 

 find composed of onion-shaped gland-cells containing a nucleus 

 and granules, and provided with a usually fine duct, the 

 expanded end of which forms the hexagonal area whose 

 boundaries are, in the case of most sponges, all that has been 

 observed."^ Bidder identifies his "onion-shaped gland-cells" 

 with the cells above referred to as described by Metschnikoflf 

 in an Olynthus form, and gives a list of species in which he 

 has observed this condition. In his later papers he brings 

 forward additional evidence for believing " that in all groups 

 of sponges the flask-shaped epithelium does occur '^ (24). He 

 also identifies (24) the "pendent cell body" of his flask-shaped 

 cells with the subdermal gland-cells which have been described 

 by von Lendenfeld in the Keratosa and by myself in Grantia 

 labyrinthica. This identification is probably correct, but I 



' It appears to me that it is generally much more easy to see the nuclei 

 than it is to see the outlines of the cells. 



