90 The Echinus or Sea Urchin. 



according to the age of the animal, and arranged in very exact and 

 beautiful order. All the plates are of a similar shape, and there are 

 twenty vertical rows of them, stretching from the upper to the 

 lower aperture, arranged in pairs. In one pair each plate will be 

 observed to be pierced with six holes, and consequently these two 

 rows will form that part of the corona containing one of the radiating 

 lines of dots, alluded to before. The plates in the next pair will be 

 observed to be covered with a very regular arrangement of elevated 

 points or bosses — those bosses not scattered indefinitely over the 

 surface, but in a uniform order; consequently, when the eye sees the 

 mass of bosses before it, though from their number they appear to 

 be without any regular order, a little inspection will show that not 

 only in this, but in every point observable about the echinus, there is a 

 a very beautiful symmetry and regularity. The corona of the echinus 

 is therefore made up of ten tracU or spaces, fiive containing holes and 

 five elevated bosses, the former are called the ambulacral tracts, the 

 latter the inter-ambulacral. 



Of the two holes, the upper one, at the apical pole (and therefore 

 called the apical aperture) , is in the living animal almost entirely occu- 

 pied with ten plates of a difierent shape from those forming the corona, 

 five of these, surrounding the small aperture at the top, are called 

 genital plates, and have a single hole each, attached to which, at the 

 proper season of the year, is a large bag of orange-coloured eggs, es- 

 teemed a great delicacy in some countries, especially those surrounding 

 theMediterranean, and for which alone large quantities of these Echini 

 are caught and brought to market. The other five plates are called 

 ocular plates, and have also a single spot in each, considered to be an 

 eye, this being the only organ of sense hitherto detected in these very 

 low and yet very complex organisms. These ten plates are so arranged 

 that an ocular plate is situated at the top of one of the ambulacral areas, 

 and a genital at the top of one of the inter-ambulacral. One of the 

 genital plates is difierently formed from the other four, presenting a 

 more spongy appearance : this will be alluded to further on — it is 

 called the madriporiform tubercle. 



The other hole, called the oral aperture, is very nearly covered 

 with a tough membrane, in which are numerous small calcareous 



