12 Mr. H. J. Carter on the Anatomy 



the central canal of the spicule is formed ; and upon the 

 tubular prolongations as they extend is deposited, in concentric 

 layers, the siliceo- or calcareo-albuminous material of which the 

 spicule may be composed, the extremity of the tubule or 

 central canal only becoming covered when the fundamental 

 form of the spicule is completed. Hence the spicule always 

 has a central canal, which remains hollow in the siliceous ones, 

 but in the calcareous spicules appears to me to be filled up 

 by the same material of which the spicule itself is composed ; 

 while in some large, robust, acerate siliceous spicules, too, it 

 is often diminished to an almost imperceptible line in the 

 centre, although comparatively wide towards the extremities — 

 thus showing, in some instances, a tendency to become filled 

 up in the same way as the calcareous spicules. 



That the spicule is developed from a central cell is often 

 confirmed by the presence in some sponges of more or 

 less abortive attempts at elongation, whereby globular or 

 elliptical bodies of considerable size are formed through the 

 deposit of concentric or successive layers of siliceo-albuminous 

 material upon a central or elongated cell as the case may be, 

 which for some reason has remained stationary, although it 

 has continued to develop successively the layers of which the 

 normal linear form is composed. 



When once the spicule can be recognized, it is not difficult 

 to follow its further development, which goes on pari passu 

 with the extension of the central canal, linearly or in a radiating 

 manner, as before noticed. If the spicule has a decided linear 

 shaft, this makes its appearance first, and the radiating branches 

 appear afterwards at one of its extremities ; so that iheprimary 

 form of a shafted spicule would always be a straight line. 

 At least this is what may be seen among the spicules in the 

 ovum of Tetliya cranium (' Annals,' 1872, vol. ix. p. 429, 

 pi. xxii. fig. 16). But while the central canal goes on ex- 

 tending itself as the spicule grows larger, it never goes beyond 

 what may be termed the fundamental form of the spicule, 

 which is thus determined by the central canal. All orna- 

 mental or subsidiary parts, such as the spines &c, are sub- 

 sequently added, probably after the spicule has left the mother 

 cell and has got into the intercellular sarcode, as shown by 

 the central canal never extending into them. But still it may 

 be a question whether they are not all initiated by the central 

 canal, and thus appear to be evolved like any other develop- 

 ment which cannot be traced backwards beyond a certain 

 point. 



We shall find by-and-by that, besides the spicules especially 



