The Difference Between...Regular and Irregular Echinoids

Toxaster peroni an example of an irregular echinoid exhibiting
bilateral symmetry.

An echinoid is a sea urchin, we find the fossils of the echinoids from the Ordovician to the present day. The two types, irregular and regular, have few similarities and it is relatively easy to identify which is which.

Both echinoids live in marine environments, their anatomy have some similarities as well, they have five ambulacrum and have a mouth and anus. They also use their tube feet for respiration. But this is where the similarities end.

Regular echinoid from the family Toxopneustidae. Note the five
fold symmetry. Image credit nhm.ac.uk

The lifestyles differ completely, irregular echinoids, like the Toxaster peroni pictured right, are infaunal echinoids, meaning they burrow into the sediment. They also favour low energy environments. Regular echinoids are found in the opposite conditions, being epifaunal they lie on the sea bed and prefer high energy conditions.

The feeding habits of the echinoids are different, the regular echinoids, being epifaunal, graze on the sea bed using the Aristotle's Lantern. This is the chewing organ of the sea urchin that is composed of five calcium carbonate teeth and an internal tongue. On the other hand the irregular echinoid particle feeds by filtering the water.

The tube feet have different functions for each echinoid, both use them for respiration but there are more uses. The regular echinoids use them to attach themselves to the sea bed and also to move. This shows that the regular echinoids are more active than the irregular echinoids. The irregular urchins use their tube feet for digging their burrows and maintaining them.

When looking at the fossils it is easiest to identify which is which by looking at the symmetry. Regular echinoids have five fold symmetry due to being a lot more round, whereas irregular echinoids have bilateral symmetry. It is easy to see this in the images.