Chap. !.] TEST OF ECHINOIDEA. 289 



of an organic basis, which becomes impregnated with 

 calcareous salts, and, in thin sections, has a ver} 

 characteristic reticular appearance. 



It is particularly well developed intheEchinoidea, 

 with the consideration of which it will be convenient 

 to commence. In recent forms the test ^corona) 

 is made up of ten pairs of rows of plates, five of which 

 are radial and five interradial in position; the 

 former are perforated at the outer edge to allow of the 

 passage of the ambulacral tubes or suckers ; in the 

 fossil Palcechinoidea the interambulacral plates were 

 not paired, but as many as five or six took the place 

 of the two which are now constantly developed in all 

 known living species. These plates of the corona, 

 which are covered by an epithelial lining and by the 

 extracoronal portion of the peripheral nervous 

 system, are ordinarily firmly attached to one another, 

 so that no part of the corona is movable ; in some, 

 however, such as Asthenosoma, the plates are mov- 

 able on one another, and the whole test is flexible. 



The rows of pores may remain straight, as in Cidaris, 

 or three or more primary may unite to form larger 

 secondary plates, and the pores then become arranged 

 in arcs ; three pairs of pores go to form an arc in an 

 Echinus, and as many as twelve or thirteen in a 

 Heterocentrotus. The plates carry tubercles of vary- 

 ing sizes, and on these tubercles (Fig. 121; B) are 

 placed movable spines, which may be quite short, as 

 in Echinus, longer than the long axis of the body, as 

 in the piper (Dorocidaris), or very strong and massive, 

 as in Heterocentrotus. Sometimes, as in Diadem a, 

 these spines are not only protective organs in virtue of 

 their own strength and number, but are also capable 

 of inflicting painful burning wounds in a manner which 

 has not yet been satisfactorily explained. Sometimes, 

 as in Spatangus or Echinocardium, the spines become 

 very fine and silky. In most, though not in all cases 

 T 16 



