710 



ON CONCRETIONS, SPICULES, ETC. 



[CH. 



we have others in which the accumulating peUicles of skeletal matter 

 have extended from the edges into the substances of the boundary 

 walls and have so produced a system of films, normal to the surface 

 of the sphere, constituting a very perfect honeycomb, as in Ceno- 

 sphaera favosa and vesparia*. 



In one or two simple forms, such as the fresh-water Clathrulina, 

 just such a spherical perforated shell is produced out of some 



Fig. 324. Ethmosphaera conoslphonia Fig. 325. Portions of shells 



Hkl. of two "species" of 



Cenosphaera : upper 



figure, C. favosa ; lower, 



C. vesparia Hkl. 



organic, acanthin-like substance; and in some examples of Clath- 

 rulina the chitinous lattice-work of the shell is just as regular and 

 delicate, with the meshes for the most part as beautifully hexagonal 

 as in the siHceous shells of the oceanic Radiolaria. This is only 

 another proof (if proof be needed) that the peculiar form and 

 character of these little skeletons are due not to the material of 

 which they are composed, but to the moulding of that material 

 upon an underlying vesicular structure. 



Let us next suppose that another and outer layer of cells or 

 vesicles develops upon some such lattice- work as has just been 



* In all these latter cases we recognise a relation to, or extension of, the principle 

 of Plateau's bourrelet, or van der Mensbrugghe's masse annulaire, or Gibbs's ring, 

 of which we have had much to say. 



