Major subgroups in the phylum

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.

CLASS ECHINOIDEA (Echinoids)

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:

Infraclass Echinothurioidea. Deep-sea echinoids with flexible test.

Infraclass Acroechinoidea. Upright lantern and deep foramen magnum, compound ambulacra, peristomial tube feet. Includes three cohorts:

Cohort Diadematacea. Many primitive features retained. Grooved teeth.

Cohort Echinacea. Advanced features such as keeled teeth. Compound plates and complex perignathic girdle. TWO ORDERS, distinguished on lantern structure:

  • Order Stirodonta. Example: Acrosalenia
  • Order Camarodonta. Example: Echinus

Cohort Irregularia. All IRREGULAR echinoids. All those that retain the lantern have diamond-shaped teeth. A number of subgroups recognised:

Superorder Eognathostoma. 'Intermediate' genera from which members of Superorder Microstomata are thought derived. Includes:

  • Order Pygasteroida. Periproct half-way out of the apical disc. Example: Pygaster.
  • Order Holectypoida. Periproct fully disjunct, other features remain quite simple.

Superorder Microstomata. Periproct fully disjunct. Lantern often lost in the adult. Divided into:

Neognathostomata. Rounded, pentagonal or elliptical outline. Retain lantern, in juveniles at least. Includes:

  • Order Cassiduloida. Subglobular. Example: Catopygus.
  • Order Clypeasteroida. Flattened, the most recent order of echinoids. Example: Mellita.

Atelostomata. Heart-shaped urchins, lantern lost. Includes:

  • Order Disasteroida. Split apical system.
  • Order Holasteroida. Elongated apical disc.
  • Order Spatangoida. Compact apical system and features seen in Example: Echinocardium

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.

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