ECHINODERMA TA—ECHINOIDEA . 



573 



upon the membrane. Muscles connect the pyramids with each 

 other and with projections from the soHd test surrounding it. 



The anus is surrounded by a soft membrane similar to that 

 around the mouth and is placed either at the center of the apical 

 system or at a variable distance from it in the median line of the 

 posterior interambulacrum, upon either the upper or the under 

 surface of the test. With mouth centrally placed and anus at 



iamb 



Fig. 191 1. Diagram of Linihia variabilis. A, dorsal view ; B, posterior view ; 

 C, ventral view. (After A. W. Slocum.) «, apical system ; amb, ambulacra ; as, 

 anterior sulcus ; iantb^ interambulacra ; If, lateral fasciole ; /, petaloid part of ambula- 

 crum ; />/, peripetalous fasciole ; //, periproct ; ps, peristome. 



the center of the apical system, the test is said to be regular or 

 tndocyclic; with mouth central or excentric, and anus excentric, 

 the test is known as irregular or exocycUc. 



The plates of the test are arranged in ten meridian-like zones. 

 Five of these, the amhulacral areas, are composed of small per- 

 forated plates, the remaining five inter ambulacral areas alternate 

 with these and are imperforate and usually larger. In the vast 

 majority of echinoids, each zone is composed of two columns of 

 plates, making twenty columns in all, ten perforate, and ten non- 

 perforate; this number is, however, not attained in some fossil 

 forms and is exceeded in others. 



Interambulacral (interradial) plates are always simple; ambu- 

 lacral plates may be either simple or compound. When compound 

 they may be formed of two or of more parts, all of which are 

 joined by sutures and form a more or less geometrical plate. 



New plates are successively added to the ends of the ambulacra 

 and interambulacra next to the apical system. 



Each ambulacrum consists of an interporiferous area placed 

 between two poriferous zones; only a few Palaeozoic genera have 

 the whole ambulacral area pore-bearing. Usually the ambulacral 

 pores are in pairs. The arrangement of the pairs of pores may be 



