442 



ON CONCRETIONS, SPICULES, ETC. 



[CH. 



a plane C-shaped figure, but is discovered, on more careful inspec- 

 tion, to lie not in one plane but in a more complicated spiral twist. 



Fig. 212. 



Fig. 211. Sponge and Holothurian spicules. 



This investigation includes a series of forms which are abundantly 

 represented among actual sponge-spicules, as illustrated in 

 Figs. 211 and 212. If the spicule be not restricted 

 to Unear growth, but have a tendency to ex- 

 pand, or to branch out from a main axis, we shall 

 obtain a series of more complex figures, all related 

 to the geodetic system of curves. A very simple 

 case will arise where the spicule occupies, in the 

 first instance, the axis of the containing cell, 

 and then, on reaching its boundary, tends to branch or 

 spread outwards. We shall now get various figures, in some 

 of which the spicule will appear as an axis 

 expanding into a disc or wheel at either 

 end; and in other cases, the terminal disc 

 will be replaced, or represented, by a series 

 of rays or spokes, with a reflex curvature, 

 corresponding to the spherical or ellipsoid 

 curvature of the surface of the cell. Such 

 spicules as these are again exceedingly 

 common among various sponges (Fig. 213). 

 Furthermore, if these mechanical methods 

 of conformation, and others like to these, 

 be the true cause of the shapes which the 

 spicules assume, it is plain that the pro- 

 duction of these spicular shapes is not a specific function of 

 sponges or of any particular sponge, but that we should expect 



Fig. 213. All " ampliidisc " 

 of Hyalonema. 



