The echinoids belong to the Phylum Echinodermata, all members of which are marine. The group is generally divided into stalked (pelmatozoan) and mobile (eleutherozoan) forms and these are divided into two main subphyla. See explanation for terms relating to classification (and the fossil record) which may be unfamiliar.
Subphylum Pelmatozoa includes the crinoids (sea lilies and feather stars) and a number of extinct groups. Subphylum Eleutherozoa includes one extinct group and the living asteroids (starfish), ophiuroids (brittle stars), holothuroids (sea cucumbers) and echinoids (sea urchins, heart urchins and brittle stars). The echinoids, Class Echinoidea, will now be given further consideration.
A number of different classifications have existed. One of the debated issues has been whether the irregular echinoids are monophyletic or polyphyletic: recent cladistic analyses indicate that they form a monophyletic group (explanation).
A division into three subclasses is currently recognised. This is based on the lantern structure, plating of the peristome, and arrangement of ambulacral and interambulacral plates. Outline drawings are given of some typical echinoids below the classification table to illustrate their diversity.
Subclass Perisoechinoidea (Late Ordovician-Permian, i.e. Palaeozoic)
All are regular with 2-20 columns is ambulacra and many columns in interambulacra. Lantern with simple grooved teeth and perignathic girdle simple or absent. Example: Aulechinus
Subclass Cidaroidea (Devonian-Recent)
All are regular with only two columns of plates in ambulacra and interambulacra of later forms. A large tubercle on interambulacral plates supports a massive spine. Lantern with no foramen magnum, perignathic girdle has apophyses only. Example: Cidaris
Subclass Euechinoidea (Upper Triassic-Recent, i.e. post-Palaeozoic)
Both regular and irregular. Two columns of plates in both ambulacra and interambulacra. Further subdivided into TWO INFRACLASSES:
Order Stirodonta: Acrosalenia
Order Camarodonta: Echinus
Order Pygasteroidea: Pygaster
Order Cassiduloida: Catopygus
Order Clypeasteroida: Mellita
Order Spatangoida: Echinocardium
An excellent website, The Echinoid Directory, set up by Andrew Smith of the Natural History Museum, provides a resource for taxonomic identification of echinoids. The directory itself is aimed at the scientific community and contains a lot of jargon but worth a look for the lovely photographs.