IX] 



AND SPICULAR SKELETONS 



413 



and the other calcareous or "stony" algae), appears in the form 

 of fine rounded granules, whose inherent crystalline structure 

 is not outwardly visible, but is only revealed (like that of a 

 molluscan shell) under polarised light. Among animals, a skeleton 

 of carbonate of lime occurs under a multitude of forms, of which 

 we need only mention now a very few of the most conspicuous. 

 The spicules of the calcareous sponges are triradiate, occasionally 

 quadriradiate, bodies, with pointed rays, not crystalline in outward 

 form but with a definitely crystalline internal structure. We shall 



Fig. 194. Alcyonarian spicules: Siphonogorgia and Anthogorgia. (After Studer.) 



return again to these, and find for them what would seem to be 

 a satisfactory explanation of their form. Among the Alcyonarian 

 zoophytes we have a great variety of spicules*, which are some- 

 times straight and slender rods, sometimes flattened and more or 

 less striated plates, and still more often rounded or branched 

 concretions with rough or knobby surfaces (Figs. 194, 200). A 

 third type, presented by several very different things, such as 

 a pearl, or the ear-bone of a bony fish, consists of a more or less 



* Cf. Kolliker, Icones Histiologicae, 1864, pp. 119, etc. 



