162 HALF AN HOUR WITH SEA-URCHINS 



that of the teeth of higher animals. The keel of these 

 teeth, to which we have referred, is composed of rods 

 of carbonate of lime, which lie oblique to the axis of 

 the tooth. The chisel -shaped edge consists first of 

 a series of triangular, calcareous plates, called 

 "primary plates." These constitute a framework 

 with which the other parts become connected. To 

 these plates, at some distance from the base, are 

 attached a series of lappet-shaped laminae, to which are 

 added reticulations of limy fibres, having a fan-shaped 

 termination. Altogether, therefore, it will be seen 

 that the structure of the teeth of sea-urchins is 

 really very complex. In Fig. 84 we have a longi- 



Fig. 85. 



m&gm 



Longitudinal Section of Tooth. Portion of Shell of Echinus 



lividas, X 230. 



tudinal section, showing the manner in which the 

 minute plates of lime interlap each other. 



The microscopical structure of the shell-case, or 

 " test," of the sea-urchin is no less interesting. We 

 have spoken about the number of pieces which are 

 mosaicked together to form this test, and in Fig. 85 



