VIII 



EOHINODEBMA TA SPINES, ETC. 



389 



fibres, which latter bring about the movements of the spine on the 

 tubercle. Both the muscular, and the elastic, fibres are attached on the 

 one hand to the socket (below the circular ridge or cushion), and on 

 the other to the area surrounding the tubercle. They pass into the 

 organic substance of the skeleton. 



The spine is covered from tip to base (i.e. to the neck) by a very 

 hard and thick calcareous layer, the cortical layer, which, in the 



Fin. 33C>. Part of the 

 surface of the test of 

 Cidaris tribuloides, Ag., 



near the ambitus, to show 

 the tubercles and ambu- 

 lacral pores up. ia, Interam- 

 bulacral row of plates ; am, 

 ambulacral row of plates. 



FIG. 337. Large spine of a Cidaris, dia- 

 grammatic (essentially after Prouho). 1, Cor- 

 tical layer ; 2, middle layer ; 3, medulla ; 

 4, neck ; 5, integument ; 6, pedicle ; 7, axial 

 band ; 8, muscle ring ; 9, circular ganglion ; 

 10, ligamentous envelope ; 11, tubercle on 

 the test ; 12, test. 



development of the test, is the last part deposited, and determines 

 the ornamentation of the spine. 



At first the body integument covers the whole spine, its epithelium 

 being provided with cilia. But when the spine has attained its 

 definitive size, and the cortical layer has been formed, the integument 

 dies away on those parts which are covered by that layer, and is only 

 retained round the base of the spine. 



At this base, about half way' up the muscular envelope, under the 

 surface of the epithelium, lies a nerve ring with scattered ganglion 

 cells ; this runs right round the base of the spine, and innervates its 

 muscles. 



