IV. ECHINOIDEA 



303 



Sub Class III. CyslMea. 



Exclusively paleozoic; body spherical, composed of polyi^onal plates. Stalk 

 and arm structures rudimentary, sometimes lackini,'. The .\mphorida are 

 Vjy some regarded as ancestral of all echinoderms. liolocystites, Echino- 

 spharites (fig. 299). 



Fig. 



299. 



Fig. 



Fig. 

 Fig. 



299. — Echinosphcrriles aiiranlium (from Zittel). 



300. — FeiUremiles Jiorcalis (from Zittel). Lateral, oral, and aboral views. 



Siib Class IV. Blastoidca. 



Arms lacking; the mouth surrounded by five petaldike ambulacral areas. 

 The group appears at end of Silurian and dies out with the carboniferous. 

 Penlremiles (fig. 300). 



Class IV. Echinoidea (Sea Urchins). 



The structure of the sea urchins is best understood in the spherical 

 forms (figs. 301, 303). Mouth and anus lie at opposite poles of the main 

 axis, each opening immediately sur- 

 rounded by areas covered by calcare- 

 ous plates, the arrangement of which 

 varies with the family. Around the 

 anus is the periproct, around the mouth 

 the peristome, the latter bearing sphas- 

 ridia and in the Echinoids five pairs 

 of interaml)ulacral gills. Between 

 peristome and periproct the body wall 

 {corona) is composed of calcareous 

 plates, which, except in the Echinothu- 

 rida;, are immovably united. Aside 

 from the extinct Patechinoidea the 

 plates are arranged in ten double 

 meridional rows, two rows being 

 always intimately associated together. 

 Five of these double rows are ambulacral, the alternating five interam- 

 bulacral. Both bear small hemispherical articular surfaces on which 



Fig. 301. — Cceloplninis fioridanus* 

 (after Aga.ssiz). Aboral \'ie\v, the 

 spines removed to show the ambula- 

 cral (a) and (/)) interambulacral areas, 

 ending respectively in the ocular and 

 genital plates; in the centre the four 

 plates of the periproct. 



