The Echinoid Directory

Key to the major groups of Euechinoidea

1a. Columns of ambulacral plates with tube-feet extend across the peristomial membrane. Test plating imbricate


1b. Peristomial membrane with just five pairs of buccal plates bearing tube-feet (arrowed). Test plating rigid

Acroechinoidea - Go to 2

2a. Lantern with teeth that grooved on their inner surface (arrow - internal of hemipyramid illustrated)

Go to 3

2b. Spines solid. Lantern with teeth that are T-shaped in cross-section (arrow - internal of hemipyramid illustrated).

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3a. Apical disc dicyclic (ocular plates not forming part of periproct rim). Spines solid with dense central mesh.


3b. Apical disc monocyclic to hemicyclic (ocular plates forming part of periproct rim). Spines hollow or with open mesh fill.


4a. In lantern, epiphyses small and slender; not fused above foramen magnum.
4b. In lantern, epiphyses solid and fused above foramen magnum.