Irregularia

Irregularia Temporal range:
Echinocardium cordatum
Scientific classification
Kingdom:	Animalia
Phylum:	Echinodermata
Class:	Echinoidea
Infraclass:	Irregularia
superorders
Atelostomata Neognathostomata

The Irregularia are an infraclass of sea urchins.

Description and characteristics

These particular sea urchins are distinguished from other sea urchins by their irregular shape : the anus and often even the mouth are no more at the two poles of the test, creating a bilateral symmetry instead of the classical 5-fold symmetry of echinoderms. The group includes the well known heart urchins, as well as flattened sand dollars, sea biscuits and some other forms. Most of them live inside the sediment, movin in thanks to their particular spines, and feed on its organic fraction.

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  Test of a spatangoid (oral face) : the mouth is on the right and the anus on the left.
• 
  Echinocyamus pusillus.
• 
  Test of a Mellita quinquiesperforata.
• 
  A "sand dollar" in its environnement.

• 
  Clypeaster reticulatus (family Clypeasteridae).
• 
  Encope emarginata (aboral and oral faces) drawn by Ernst Haeckel (1904).
• 
  Clypeaster rosaceus, ibid.
• 
  Many slices of a Clypeaster aegypticus, showing buttresses and pillars.

Taxonomy

Fossil of an Hemipneustes pyrenaicus (Holasteroida).

Fossil of a Conulus subroundatus (Echinoneoida).

Fossil of a Scutella subrotunda (Clypeasteroida).

Fossil of an Echinolampas hemisphaericus (Echinolampadoida).

• super-order Atelostomata
  • fossil familia Acrolusiidae Mintz, 1968 †
  • fossil familia Collyritidae d'Orbigny, 1853 †
  • fossil familia Disasteridae Gras, 1848 †
  • order Holasteroida
  • order Spatangoida
  • fossil familia Tithoniidae Mintz, 1968 †
• fossil genus Atlasaster Lambert, 1931 †
• fossil familia Desorellidae Lambert, 1911a †
• order Echinoneoida
• fossil familia Galeropygidae Lambert, 1911a †
• fossil genus Grasia Michelin, 1854 †
• fossil order Holectypoida †
• fossil familia Menopygidae Lambert, 1911 †
• fossil genus Mesodiadema Neumayr, 1889 †
• super-order Neognathostomata
  • familia Apatopygidae Kier, 1962
  • fossil familia Archiaciidae Cotteau & Triger, 1869 †
  • order Cassiduloida
  • order Clypeasteroida
  • fossil familia Clypeidae Lambert, 1898 †
  • fossil familia Clypeolampadidae Kier, 1962 †
  • order Echinolampadoida
  • fossil familia Nucleolitidae L. Agassiz & Desor, 1847 †
  • fossil familia Pygaulidae Lambert, 1905 †
  • fossil genus Pygolampas Saucède, Dudicourt & Courville, 2012 †
• fossil familia Pygasteridae Lambert, 1900 †
• fossil familia Pygorhytidae Lambert, 1909b †

References

• Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. p. 981. ISBN 0-03-056747-5.
• Kroh A, Mooi R (eds.). "Irregularia". World Echinoidea Database. World Register of Marine Species.