294 



ERYLUS ROTUNDUS. 



composed of polyedric parts. It is possible that the spaces between these cells 

 and the walls of the cavities, within which they lie, and which appear to be empty, 

 are in reality thick, hyaline, cell walls. But as these spaces are not stainable 

 with any of the stains (eosin, malachite-green, magenta, aniline-blue, methyl- 

 violet, azure, haematoxylin, aurantia, Bismarck-brown), I think this improb- 

 able. The cells composed of parts are much more numerous than the simple, 

 undivided ones. The latter are scattered quite irregularly between the former 

 and do not increase in number either towards the surface or towards the interior. 

 Although convinced that the undivided ones and the ones composed of polyedric 

 parts are merely different stages in the development of the same kind of cell, 

 I am unable to say whether the simple ones arise from the composed ones or 

 vice versa. 



In the sections of var. typica, form B, groups of broad, irregularly oval cells, 

 28-32 n long, were observed in the distal part of the choanosome. The plasm 

 of these cells is granular and each one contains a large nucleus, about 8 /< in 

 diameter. These cells appear to be ova. 



In the sections of var. cidaris young larvae were observed. Some of these 

 lay free on the canals, others appeared to be just on the point of emerging from 

 the cavities of the ground substance in which they were bred. These larvae 

 are spherical, measure 50-60 [i in diameter, and appear to consist of a central 

 granular mass, surrounded by a single layer of roundish, not elongate cells 

 about 8 n in diameter. 



In the choanosome of var. typica, form D, large numbers of monocellular 

 symbiotic Algae were observed. These are spherical or oval, measure 15-20 p. in 

 maximum diameter, and have a stout cell wall about 4 p thick. 



Canal-system. The uniporal entrances to the canal-system are usually 

 circular, and, when quite open, 100-250 p wide. Dilated pores of this width 

 are however not frequent, most of the pores being more or less contracted and 

 smaller, or closed altogether. The flagellate chambers are more or less spherical 

 and measure 14-23 p in diameter. Those of var. cidaris are smaller (diameter 

 14-17 p) than those of the others. The collar cells clothing them are not numer- 

 ous, distant, rather slender, and 4-6 p long. The larger canals are surrounded by 

 stout mantles of tissue free from megascleres and flagellate chambers. Some of 

 them are traversed by sphincter-membranes. Such have been particularly 

 observed in var. typica, form B. In the forms C and D of var. typica and in var. 

 cidaris no apertures much larger than the pores described above, were observed 

 on the surface. In these sponges the efferent openings (oscules) do not seem 



