The Echinoid Directory

Key to the major clades of Irregularia Latrille

1a. Peristome relatively large, central and circular with buccal notches. Ambulacra with simple undifferentiated pore-pairs throughout


1b. Peristome generally small and always lacking buccal notches. Ambulacra with differentiated tube-feet and pore-pairs (forming phyllodes and/or petals)

Go to 2

2a. Plating of all five interambulacral zones on the oral surface similar

Neognathostomata - Go to 3

2b. Posterior interambulacral zone on oral surface differentiated into a plastron


3a. Single pore-pair to each ambulacral plate beneath petals. Pore-zones usually expanded close to peristome

Cassiduloida and stem group Neognathostomata

3a. Many pore-pairs to each ambulacral plate beneath petals. Pore-zones never expanded close to peristome