IV ECHINOIDEA 



303 



Sub Class III. Cystidea. 



Exclusively paleozoic; body spherical, composed of polygonal plates. Stalk 

 and arm structures rudimentary, sometimes lacking. The AMPHORIDA are 

 by some regarded as ancestral of all echinoderms. Holocystites, Echino- 

 splucnles (fig. 299). 



V< 



FIG. 299. FIG. 300. 



FIG. 299. Ecliinosplnrrites aurantium (from Zittel). 



FIG. 300. Pentremites florealis (from Zittel). Lateral, oral, and aboral views. 



Sub Class IV. Bias to idea. 



Arms lacking; the mouth surrounded by five petal-like ambulacral areas. 

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

 Pentremites (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 peristoine, the latter bearing sprue- 

 ridia and in the Echinoids five pairs 

 of interambulacral gills. Between 

 peristome and periproct the body wall 

 (coron'.i) is composed of calcareous 

 plates, which, except in the Echinothu- 



ridce, are immovably united. Aside / IG .^.-C^oplcnrus ./?,/,/<,,* 



(after Agassiz). Aboral view, the 



from the extinct Pakechinoidea the spines removed to show the ambula- 



plates are arranged in ten double oral (a) and (6) interambulacral areas 



endme; respectively m the ocular and 



meridional TOWS, two rows being genital plates; in "the centre the four 



always intimately associated together. P lates of the periproct. 

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

 bulacral. Both bear small hemispherical articular surfaces on which 



