ECHINOIDEA. 



367 



in many cases is placed at the summit of the test, and is surrounded 

 by the so-called "apical disc" or " dorso-central system"; but it is 

 in other cases marginal or submarginal, being then separated from 

 the apical disc, and being almost always situated in the posterior 

 interradius. In Echinocystites (Cystocidaris) alone the anus is ex- 

 centric and interradial, but is placed near the apex. 



The radial nerve-cords run along with the radiating ambulacral 

 vessels in the inside of the shell, and become connected at their 

 terminations with a series of perforated plates (" ocular plates ") 

 which form part of the so-called " apical disc," and which are " radial " 

 in position. The generative glands occupy the " interradial " areas, 

 and their ducts normally open by perforations in the so-called "gen- 

 ital plates," which likewise form part of the " apical disc." 



It is necessarily principally with the test of the Echinoids and its 

 appendages that the palaeontologist is concerned, and these struc- 

 tures must therefore be considered in some detail. The test of the 

 Echinoidea may be regarded as essentially composed of the so-called 



-Echinoidea. Test of Echinus esculentus, viewed from above, a, One of the 

 ambulacral areas ; ia, One of the interambulacral areas. 



" corona " and of the " apical disc " ; though minor and less constant 

 plates are likewise developed in the membranes surrounding the 

 mouth and anus respectively. 



The " corona " forms the main element of the test, and is com- 

 posed of numerous calcareous plates, more or less firmly united to 

 one another by their edges, arranged in rows, and bearing different 



