The Echinoid Directory

Key to the major clades of Echinoidea

1a. Test composed of more than 20 columns of plates (more than 2 ambulacral or interambulacral columns in each zone)

Stem group (Palaeozoic) echinoids

1b. Test composed of just 20 columns of plates; interambulacral and ambulacral zones each composed of two columns of plates

Go to 2

2a. Test with bilateral symmetry superimposed over pentaradial plan; periproct displaced to the posterior - almost always lying outside the apical disc plates.


2b. Pentaradially symmetric; periproct opening surrounded by circlet of apical disc plates

Go to 3

3a. Perignathic girdle rudimentary or absent.

Advanced stem group Echinoidea, basal stem group Cidaroidea or Euechinoidea

3b. Perignathic girdle well developed, composed of ambulacral auricles and/or interambulacral apophyses.

Go to 4 ('Regular' echinoids)

4a. Perignathic girdle always present and composed of interambulacral apophyses (ap) only. Ambulacral plating simple and extending onto the peristome.


4b. Perignathic girdle composed of ambulacral auricles (au). Ambulacra always compound.