398 Miss I. B. J. SoUas on the Inclusion 



sections of selected bits of cortex which, owing to their 

 comparative freedom from sand-contents, could be cut without 

 desilicification. Icannot doubt that theinclusionof sand-grains 

 is due to the activity of these small granular ama3bocytes ; 

 and I think it is worth while to consider whether some similar 

 cellular activity is not responsible for tlie inclusion of foreign 

 bodies in sponges generally, for, on the hypothesis that this 

 phenomenon is a property of the spongin-fibre, the fact of 

 selection of particular kinds of foreign bodies by various 

 species remains unexplained. Why, for example, do fibres 

 of Phyllospongia silicata contain foreign spicule fragments, 

 while in various other species of Phyllospongia sand-grains 

 are found as the fibre-core ? or, again, in IRppospongia why 

 siiould one group of species possess fibres free from foreign 

 bodies, while in a second and third group the fibres contain 

 foreign spicules and sand-grains respectively ? It seems to 

 me still more difficult to account for the skeleton of some 



Fig. 3. 



Su 





Thin section of the desilicified cortex of Migas porphyrion, 



a, partially dissolved sand-grain. 



species of Aulena without the assumption of the selection of 

 fibre-contents by cellular activity. Thus in Aulena gigantea 

 in addition to the supporting skeleton, which is cored by 

 sand-grains, there is a surface skeleton consisting of (1) sili- 

 ceous spicules lying irregularly in the "skin," (2) a tan- 

 gential network of stout fibres cored with an axial series of 

 sand-grains, (3) echinated fibres running from these to the 

 surface and cored with spicules. '' The spicules in the skin 

 are partly foreign, like those which are found in the axis of 

 the echinated fibres " (Lendenfeld's description). The 

 tangential network (2) being remote from the surface, surely 

 the sand-grains must be carried to it. 



Again, when foreign spicules are included in the fibre they 

 are arranged with thtir long axes parallel to the axis of the 



