AND STAB-FISH. 163 



we give our readers an illustration of a magnified 

 portion of one of these pieces, as it may be seen in 

 the purple echinus (Echinus lividus) of our shores. 

 Sections of the thorn-like spines of the same species 

 form really beautiful objects for the microscope. 

 Cross sections of the spines of some species look like 

 exogenous wood, and even in those of the Purple 

 Echinus this woody appearance is very striking. 



Fig. 86. 



Transverse Section of Spine of Echinus lividus, x 60. 



Just as exogenous wood has its annular rings, due to 

 the successive deposition of woody tissue formed 

 each year, so that the age of the tree may be known 

 by merely counting them ; so are the minute rings 

 seen in the section of the spines of some echini 

 evidences of the age of these animals, the lime having 

 been deposited at successive times. The spines of 

 the Purple Sea-urchin, however, are shed and re- 



