and Physiology of the Spongida. 11 



Nature of the Foreign Bodies. 



The " foreign bodies " of the core chiefly consist of grains of 

 sand mixed more or less with siliceous and calcareous spicules 

 of other sponges (entire or fragmentary), of the spicules and 

 calcareous structures of Echinodermata, of Diatomacese, and 

 of minute Foraminifera — indeed, any thing of this kind, 

 especially calcite in a minute columnar or prismatic form, 

 banded with hair-brown, yellow, and amethystine colours, 

 originally derived from the disintegration of thin bivalve 

 shells allied to Pinna. At first I was at a loss to account for 

 the origin of these little prisms ; but finding them in certain 

 kinds of sponges from all quarters of the world, especially 

 from Port Jackson in Australia, and at last in direct con- 

 nexion with some specimens of Crenula phasianoptera which 

 had been overgrown and enclosed bodily by the sponge itself, 

 their general occurrence, rhombohedral prismatic form, and 

 banded colours were thus explained. 



Spicules. 



The spicules, as their name implies, are pointed, siliceous 

 or calcareous bodies produced by the sponge itself, of an 

 infinite number of forms, varying in accordance Avith the species, 

 and extending from a simple linear one, pointed at each end, 

 to the most complicated figure. 



At first it would appear that the spicule is produced in the 

 homogeneous or intercellular sarcode (that is, the basis or 

 original living slime in which every part of the sponge is 

 developed and imbedded), as it is present and of such a large 

 size comparatively in the ovum even before the latter becomes 

 elongated into the embryonal form, as well as in the inter- 

 cellular sarcode of the adult sponge, that in either case there 

 is no cell approximately large enough to contain it. But 

 since, in some instances, it can be followed during part of its 

 development (that is, from the time it is first recognizable to 

 that in which it is considerably enlarged), while still within 

 the parent or mother cell ('Annals,' 1874, vol. xiv. p. 97, pi. x ), 

 it may be assumed that all spicules are initiated in a mother 

 cell, however soon after they may get into the intercellular 

 sarcode. Thus the spicule appears to arise, within a mother 

 cell, from a granule which, for convenience, will be termed the 

 " spicule-cell," which cell becomes extended linearly in oppo- 

 site directions, or immediately begins to put forth more or 

 less points in a radiating direction, whereby what is called 



