Irregular echinoids are characterized by having the anus outside of the apical system. The anus has moved to the edge of the test, or towards the posterior. This means that irregular echinoids are heart shaped and have bilateral symmetry. these adaptations are to allow the irregular echinoid to live in a burrow. Examples include the Cretaceous form, Micraster and the modern day sand dollar.
Mouth and adjacent area
The mouth is still on the underside of the test, but often it has move away from the centre. The mouth lacks jaws and the periphrastic girdle found in regular echinoids. Instead the animal takes in particles from sea water and filters these. There is a large lip called the labrum, projecting on the lower side of the mouth. The labrum is used to direct currents and prevent unwanted sediments getting into the mouth. Behind the labrum there is a modified set of interambulacral plates, forming the plastron. The plastron has small tubercles for attachment of spines. These small spines are used to help dig a burrow or for movement within it.
The ambulacra do not extend all the way down from the top to the mouth, but form a flower-shaped structure called the petaloid ambulacra. These have many small pore pairs for tube feet on top of the echinoid. The petaloid ambulacra at the anterior of the animal are larger than the others, and form the anterior groove. This is lined by cilia, which beat to generate currents to pass food particles to the mouth, and is called the fasciole. Very long tube feet extend from the anterior ambulacra, which are used to help dig the burrow and keep it stable
Mode of life
Irregular echinoids live in soft sediment and in a low-energy enviroment(infaunal). They do not have jaws and have a reduced sized mouth called the peristome because they filter feed. Instead they dig burrows, using the spines on the plastron.