ECHINOIDEA. 211 



anal plates are added and the central becomes indistinguishable. The 

 anus is always excentric, being displaced towards the right posterior 

 interradius (as in Asterids). 



The test or corona consists of ten double meridional rows of 

 plates passing round from the apical to the oral pole (Fig. 148). 

 Five of these double rows are 

 radial in position and constitute 

 the ambulacra, while five are in- 

 terradial and are called the inter- 

 ambulacral plates. 



This statement is true of all Euechi- 

 noidea.* In the Palaeechinoidea, how- 

 ever, the number of rows of plates in the 



interambulacra is either one (Boihrioci- _^_^ 



daris) or more than two (from 3 to 11). 



The plates are pentagonal in Fig. 150. Apical system of a young 



f i j J.T j_i Echinus (from Delage, after Lov6n). 



form and are SO placed that the An anus ; c central ; gtx one of the 



j- -t .-, genital plates. I. to V. indicates the 



median SUture between the tWO enumeration of the radii adopted in 



rows of an ambulacrum is zigzag S5b"S& STAffSS 

 (Fig. 148), while the suture sepa- * 



rating an ambulacrum from an 



interambulacrum is straight. Each plate of an ambulacrum 

 is perforated by two pores, called double pores or pore-pairs ; 

 these are placed on the side next the adjacent inter- 

 ambulacrum and give exit to the two tubes which pass from 

 each ampulla inside to each projecting tube-foot outside (Fig. 

 169). When there are more than one double pore upon an 

 ambulacral plate, as often happens, the plate is composite and 

 consists of as many plates, fused together, as there are double 

 pores upon it (Fig. 151). 



When the ambulacral plates are compound, the word primary f is applied 

 to a component which reaches right across from the ambulacro-inter- 

 ambulacral suture (outer suture) to the median suture between the two 

 rows of ambulacral plates ; occluded to a plate which reaches the median, 

 but not the outer suture ; isolated to a component cut off from both 

 sutures ; demiplate to a component which reaches the outer, but not the 

 median suture. 



The growth of the shell is effected partly by increase in size of the 

 plates already present and partly by the addition of new plates just out- 



* Except the cretacean genus Tetracidaris, in which there are four rows 

 of interambulacral plates. 



f The word primary is sometimes used for the component elements 

 of a composite plate. 



