VII 

 THE ECHINODERMATA 



WHEN a piece of limestone is broken with a hammer it 

 usually shows a smooth or granular fracture, but here 

 and there on the broken surface may often be seen 

 distinct cleavage-surfaces of calcite : these may be few 

 or many, and in particular cases may attain a large size. 

 They are the fractured surfaces of fossil echinoderms. 

 Whereas the shells of most invertebrates show on a 

 broken surface a fibrous or laminated texture, due to the 

 arrangement of an indefinite number of minute crystals of 

 calcite, it is one of the essential features of echinoderms 

 (and of no other group except the calcareous sponges) that 

 the skeleton is composed of a series of units that may 

 attain a large size, but of which every one is, mineralogi- 

 cally, a crystal of calcite not in external shape (which is 

 determined by organic secretion), but in molecular con- 

 stitution. These units have different names, according to 

 shape : the commonest are broad, flat structures, called 

 plates. In these the normal to the surface corresponds to 

 the vertical crystallographic axis. When the shape is 

 long and cylindrical, as in the radioles (articulated spines) 

 of sea-urchins, it is the long axis which is the vertical 

 crystal-axis. 



240 



