422 



ON CONCRETIONS, SPICULES, ETC. 



[CH. 



concentric ; the presence of concentric zones or lamellae, alter- 

 nately dark and clear, was especially characteristic. These 

 round " calcospherites " shewed a tendency to aggregate together 



Fig. 197. Later stages in the same experiment. 



in layers, and then to assume polyhedral, or often regularly 

 hexagonal, outlines. In this latter condition they closely resemble 



Fig. 198, A. Section of shell of Mya; B. Section of hinge- 

 tooth of do. (After Carpenter.) 



the early stages of calcification in a molluscan (Fig. 198), or still 

 more in a crustacean shell * ; while in their isolated condition 



* See for instance other excellent illustrations in Carpenter's article "Shell," in 

 Todd's Cyclopaedia, vol. iv. pp. 556-571, 1847-49. According to Carpenter, the 

 shells of the moUusca (and also of the crustacea) are "essentially composed of 

 cells, consolidated by a deposit of carbonate of lime in their interior." That is 

 to say, Carpenter supposed that the spherulites, or calcospherites of Harting, were, 

 to begin with, just so many living protoplasmic cells. Soon afterwards .however. 



