The anatomy of irregular echinoids (BMNH guide) reflects both their regular echinoid ancestry and their mode of life (BMNH guide).  Unlike the regulars, the irregulars are deposit feeders that live within the sediment.  This is a directional habit and requires a distinct front and back  to the animal.  This leads to a strong degree of bisymmetry superimposed over the ancestral penta-radial symmetry (hence 'irregular'; i.e. not 'circular').  The mouth is moved to the front of the animal and the anus to the back.  A lantern is largely redundant and has been lost in most groups.  Defence is less of an issue within the sediment, and the spines have become highly reduced and specialised towards burrowing and circulation within the burrow (BMNH guide).

* Adapical * Fasciole * Periproct
* Adoral * Genital Plate * Peristome
* Ambulacra * Genital Pore * Petal Ambulacra
* Anterior Ambulacra * Interambulacra * Pore Pairs
* Anus * Mamelon * Spines (Also)
* Apical Plates * Mouth * Test
* Areole * Ocular Plate * Tubercles



1).  Adapical (uppersurface) views of recent irregular echinoid tests (shells); (A) With the covering of spines; (B) without.  The ancestral penta-radial symmetry is reflected in the five columns of ambulacra, with interambulacral areas in between.  The ambulacra bear the fleshy tube-feet, which extend from the test and aid in feeding and respiration.  The anterior ambulacra, at the front of the animal, is depressed for channelling food to the mouth and bears elaborate tube-feet which collect food from the sediment surface.  The other four ambulacra are termed petals.  The interambulacral area are covered in tubercles which bear spines for burrowing.  The fascioles bear highly reduced and specialised spines which circulate material within the burrow.  The apical plates consist of an assymetrical ring of genital and occular plates.  Each genital plate bears a genital pore through which reproductive fluids are released. Together, the ocular and genital plates form the oculogenital ring.  A specialised genital plate (the madreporite) is sieve-like, and is involved in the supply of water to the hydrostatically-operated tube-feet.


2).  Adoral (undersurface) view of a recent irregular echinoid.  The peristome is indicated; a flexible plated surface in life which encloses the mouth-opening.  This is directed towards the front of the animal for deposit-feeding.



3).  (A) Anterior (front) end of a recent irregular echinoid.  The anterior ambulacra is seen as a groove running down to the peristome, which is shaped for scooping sediment / food.  (B) Posterior (rear) end.  The periproct is a flexible plated surface in life which encloses the anus.  This is located at the rear so that waste matter is left behind the animal as it passes through the sediment. 



4).  (A) Detail of a recent irregular echinoid showing the dense covering of reduced spines.  (B) Detail of the adoral surface of a recent irregular echinoid with the spines absent.  The turbercles of the interambulacra, to which the spines attach, are comprised of a bulbous mamelon with which the proximal end of the spine articulates, and an areole to which the muscle for moving the spine attaches.  The size of the areole indicates the strength of the muscles and hence how active / powerful the spines were.  The shape of the areole reflects how the spines were moved; e.g. the areoles in (B) are skewed towards the bottom-right of the picture, indicating that the power-stroke of the spines was in that direction.


5).  Detail of the adapical surface of a recent irregular echinoid showing an ambulacral column.  The ambulacra are composed of two spaced rows of pore pairs.  Each pore pair was the site of one tube-foot.