208 ECHINODERMS OF THE BRITISH ISLES 



represented in British seas ; the sixth, the Holedypoidea, })eing 

 wholly extinct. 1 



Key to the orders oj Echinoids known from the British 

 {and European) seas. 



1. Anal opening inside the apical system ; test usually round ; gills 



may be present at peristomial edge (" Regularia ") . . 2 



Anal opening outside the apical system ; test usually elongate- 

 oval ; no gills at peristomial edge (" IiTCgularia ") . . 3 



2. Both ambulacral and interambulacral plates continuing over the 



peristome ; no gills at edge of peristome ; no sphaeridiae ; 

 ambulacral plates simple . . . .1. Cidaroidea 



Only the ambulacral plates (generally only one pair) continuing 

 over the peristome ; gills present at the edge of peristome ; 

 sphseridise present ; ambulacral plates mostly compound 



II. Diadematoidea 



3. Mouth central ; dental apparatus well developed ; test usually 



flattened ; tube-feet often extending also * over the inter- 

 ambulacral areas ^ . . . . III. Clypeastroidea 

 Mouth central or nearer the anterior edge of the test ; dental 

 apparatus lacking (except in the young stages of some Cassi- 

 dulids). Tube-feet confined to the ambulacra . . 4 



4. Mouth central or anterior ; unpaired posterior interambulacral area 



on under side of test not different from the other interambulacral 

 areas. Fascioles wanting . . . IV. Cassiduloidea 



Mouth anterior ; unpaired posterior interambulacral area on under 

 side of test different from the other interambulacral areas ; the 

 first plate usually widened into a sort of lip, the following pair 

 of plates either forming a broad " plastron " or displaced so that 

 one plate lies behind the other (Fig. 185). Fascioles generally 

 present ...... V. Spatangoidea 



I. Order. Cidaroidea 



Shape of test regular, round, usually spherical. Anal 

 opening in the middle of the apical area. Peristome covered 

 by regular rows of ambulacral and interradial plates (Fig. 149). 

 No gills and no gill-cuts at peristomial edge of test. Ambu- 

 lacral plates simple, never compound. Pores of the tube-feet 

 sometimes close together, separated only by a narrow, elevated 

 wall, sometimes more distant, connected by a furrow ; in " the 

 latter case they are termed " conjugate ". Each interambu- 



^ A supposed recent Holectypoid, Pygastrides relictus Loven, seems 

 rather to be a young Clypeastroid. Only a small, broken test known (West 

 Indies). 



^ The tube-feet of the interambulacral areas usually very fine and to be 

 seen distinctly only on microscopical examination. 



