436 



ON CONCRETIONS, SPICULES, 



[CH. 



The two principles of conformation are both illustrated in the 

 spicular skeletons of the Sponges. 



In a considerable number, but withal a minority of cases, the 

 form of the sponge-spicule may be deemed sufficiently explained 

 on the lines of Harting's and Rainey's experiments, that is to say 

 as the direct result of chemical or physical phenomena associated 

 with the deposition of lime or of silica in presence of colloids*. 

 This is the case, for instance, with various small spicules of a 

 globular or spheroidal form, formed of amorphous silica, con- 



^NiW^lf 



Fig. 210. Close-packed calcospherites, or so-called "spicules," 

 of Astrosclera. (After Lister.) 



centrically striated within, and often developing irregular knobs 

 or tiny tubercles over their surfaces. In the aberrant sponge 

 Astrosclera'l , we have, to begin with, rounded, striated discs or 

 globules, which in like manner are nothing more or less than the 



* In an anomalous and very remarkable Australian sponge, just described by 

 Professor Dendy (Nature, May 18, 1916, p. 253) under the name of Collosclerojyhora, 

 the spicules are "gelatinous," consisting of a gel of coUoid silica with a high 

 percentage of water. It is not stated whether an organic colloid is present together 

 with the silica. These gelatinous spicules arise as exudations on the outer surface 

 of cells, and come to lie in intercellular spaces or vesicles. 



•j- Lister, in Willey's Zoological Results, pt iv, p. 459, 1900. 



