ECHINODERMA. ECHINOIDEA 119 



fresh plates are developed next the apical disc. In each 

 area the plates alternate on either side, and since their 

 inner ends are angular, the line between the two rows is 

 zig-zag. 



The ambulacral plates are smaller and more numerous 

 than the interambulacral, and they are perforated by 

 pores (p) for the passage of the tube-feet to the exterior, 

 a radial water-vessel being placed under each ambulacral 

 area. The pores may be round or elongated ; they are 

 situated in the outer portion of the plates and are almost 

 always in pairs ; each pair of pores corresponds to a single 

 tube-foot, since each tube-foot divides at its base into two 

 canals. Frequently each pair of pores is surrounded by 

 an oval raised rim (fig. 48). In some echinoids, such as 

 Cidaris, each ambulacral plate is formed of one piece only 

 (as in fig. 46) — such plates are called simple primaries. 

 Id other cases some of the ambulacral plates are compound, 

 consisting of several small plates which have become fused 

 together ; but the original plates are still indicated by the 

 lines of suture between them and also by the pair of pores 

 on each (fig. 48) ; in some echinoids (fig. 48 A) the plates 

 which are united are all pri- 

 maries — that is to say, each 

 extends from the margin to 

 the middle line of the ambula- 

 cral area ; but frequently some Fig# 48> compound Ambulacral 



of the plates taper away and P lates - A - Pseudodiadema 



1 A hemi splicer i cum, from the Co- 



do not reach the middle line rallian, formed of three fused 



for innpr edcre of tho pom- plates. B.Phymosomakoenigi, 



^or inner eage oi me com from the chalk< formed of six 



pound plate) — Such are Called fused plates. Enlarged. 



clemi-plates {e.g. the middle plates in fig. 48 B). This 

 fusion of plates is usually stated to be due to growth- 

 pressure — since each plate of the test is enlarging and 



