ECHINIDEA. 



503 



the section was made : when a section taken from the 

 middle of the spine is examined with a power of fifty 

 diameters, it will be seen that the centre is occupied by a 

 reticulated structure ; around the margin of this may be 

 observed a series of small 

 structureless spots, ar- 

 ranged at equal distances 

 apart (Fig. 240, No. 1) ; 

 these are the ribs or pil- 

 lars, and indicate the 

 external surface of the 

 first layer deposited ; pass- 

 ing towards the margin, 

 other rows of larger pil- 

 lars may be seen, giving 

 it a beautiful indented 

 appearance ; all the other 

 parts of the section ai'e 

 occupied by the usual 

 reticulated tissue. In the 

 greater number of spines 

 the sections of the pillars 

 present no structure, in 

 others they exhibit a 

 series of concentric rings 

 of successive growth, 

 which strongly remind 

 us of the medullary rays 

 of plants ; occasionally 

 they are traversed by 

 reticulated structures, as 

 represented in Fig. 246, 

 No. 1. When a vertical 

 section of a spine is ex- 

 amined, it is found to 

 be composed of 

 placed one over the other, 

 the outer margin of each 

 cone being formed by the series of pillars. In the genus 

 Echinus the number of cones is considerable, while in that 

 of Cidaris there are seldom more than one or two ; so that 



Fig. 239. 



COneS l > Polypidom of Pennatula phosphorea. 2, 

 Synapta Chirodota. 3, Anellor-skaped 

 spiculum and plate from skin of Sy- 

 napta. 



