IX 



AND SPICULAR SKELETONS 



647 



structure; we shall 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 sometimes straight and slender rods, sometimes flattened 

 and more or less striated plates, and still more often disorderly 

 aggregations of micro-crystals, in the form of rounded or branched 

 concretions with rough or knobby surfaces t (Figs. 292, 298). A 

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



Fio;. 292. Alcyonarian sijicules: Siphonogorgia Sind AntJioyorgia. After Studer. 



})earl or the ear-bone of a bony fish, consists of a more or less rounded 

 body, sometimes spherical, sometimes flattened, in which the cal- 

 careous matter is laid down in concentric zones, denser and clearer 

 layers alternating with one another. In the development of the 

 molluscan shell and in the calcification of a bird's egg or a crab's 

 shell, small spheroidal bodies with similar concentric stria tion make 

 their appearance; but instead of remaining separate they become 



* Cf. Kolliker, Icones Histologicae, 1804, p. 119, etc. 



t In rare cases, these shew a single optic axis and behave as individual crystals: 

 W.- J. Schmidt, Arch.f. Knhr. Mech. lt, pp. .")00-.V) 1 . 1922. 



