290 COMPARATIVE ANATOMY CHAP. 



SUB-CLASS 2. Euechinoidea. 



Echinoidea with two vertical rows of plates in each ambulacral and in each 

 interambulacral area. Month on the oral side, rarely shifted towards the edge 

 (anteriorly). Teeth and jaws present or wanting. Anus either within the apical 

 system, or outside it, i.e. somewhere in the posterior interradius. 



Order 1. Cidaroida. 



Mouth central, anus within the apical system. No external gills. With jaws 

 and almost perpendicularly placed teeth. Perignathous apophysial ring interrupted. 

 Both the ambulacral and the interambulacral plates are continued over the peri- 

 stome on to the oral area as far as the mouth. On the oral area they are imbricated. 

 Ambulacra narrow. Large principal and small accessory spines. Spha?ridia want- 

 ing. Cidaris. 



Order 2. Diadematoida. 



Mouth central, anus within the apical system. So-called internal gills well 

 developed, or rudimentary, or wanting. With external gills, and incisions in the 

 peristome. With jaws and teeth. Perignathous circular apophysial ring closed. 

 Only the ambulacral plates are continued over the peristome on to the oral area, 

 where they often appear as separate buccal plates. Spha?ridia present. 



Sub- Order 1. Streptosomata. 



Test more or less flexible, with inner dorso- ventral longitudinal muscles. Both 

 external and internal gills present. The ambulacral plates (and only these) are 

 continued over the peristome on to the oral area. Fam. Echinothuridse : Pi'lmi- 

 echimis, Echinothuria, Phormosoma, Astfienosoma. 



Sub-Order 2. Stereosomata. 



Test rigid, without internal longitudinal muscles. External gills present, in- 

 ternal gills rudimentary or wanting. The ambulacral plates on the oral area are 

 replaced by isolated buccal plates. Fam. 1. Saleniidae : Peltastes, Salenia (almost 

 exclusively fossil). Fam. 2. Hemicidaridae : Hemicidaris, Acrocidaris, Gonio- 

 pygus, etc. (fossil). Fam. 3. Aspidodiadematidse : Aspidodiadema. Fam. 4. 

 Diadematidse : Diadema, Diplopodia, Pcdina, Echinothrix, Astropyga, Codechinus, 

 Orthopsis, Peronia, Echinopsis, etc. (fossil and extant). Fam. 5. Cyphosomatidae, 

 Cyphosoma, etc. (almost exclusively fossil). Fam. 6. Arbaciidse : Arbacia, Echi- 

 nocidaris (Fig. 232), Ccelopleurus, Podocidaris (extant and fossil). Fam. 7. Tem- 

 nopleuridse : Glyphocyphus, Temnopleitrus, etc. (extant and fossil). Fam. 8. 

 Echinometridae : Echinometra, Parasalenia, etc., Spongy locentrotus, Sphcerechinus 

 (mostly extant). Fam. 9. Echinidse : Echinus, Toxopneustcs, Tripneustes (extant 

 and fossil). 



Order 3. Holectypoida. 



Mouth central. Anus outside of the apical system in the posterior interradius 

 (exocyclic). With external gills. Only one pair of pores or a single pore on each 

 ambulacral plate. Jaws weak ; teeth perpendicular ; both jaws and teeth may be 

 wanting. Sphseridia present, (a) Ambulacral apophyses present: Holedypus, 



