The Echinoid Directory

1a. Apical disc plates contiguous.

1b. Apical disc disjunct.; posterior oculars not in contact with posterior genital plates.

2a. Apical disc elongate with ocular plates II and IV in contact behind genital plates 2 and 3.

2b. Apical disc compact with ocular plates II and IV separated by genital plates.

3a. Ocular plates II and IV separated only by genital plate 2.

3b. Ocular plates II and IV nestled between anterior and posterior genital plates on each side.

4a. Ocular plates II and IV housing posterior gonopores; genital plates 1 and 4 simple imperforate plates or absent.

4b. Ocular plates II and IV without gonopores; genital plates 1 and 4 housing gonopore openings.

Go to 5

5a. Posterior ocular plates border the periproctal opening; ambulacra radiating from apex of periproct.

5b. Posterior ocular plates separated from the periproct by catinal plates or interambulacral plates; ambulacra not radiating from apex of periproct.