:x] 



OF INTRACELLULAR SPICULES 



677 



The spiral bands of chlorophyll in Spirogyra, set at varying angles 

 of hehcoid obhquity, are (I take it) very beautiful examples of con- 

 tinuous growth under the restraint of a cylindrical surface. 



(Q) 



Fig. 310. Sponge and holothurian spicules. 



To return to our sponge-spicules. If the spicule be not restricted 

 to linear growth, but have a tendency to expand, or to branch out 

 from a main axis, we shall obtain a series of more complex figures, 

 all related to the geodesic system of curves. A notable 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 



Fig. 312. An "amphidisc" 

 of Ilyalonema. 



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 by rays or spokes with a reflex curvature, corresponding 

 to the spherical or elhpsoid curvature of the cell. Such spicules as 

 these are exceedingly conmion among various sponges (Fig. 312). 



