ECHINOIDS – sea urchins
Echinoids are a major group of marine invertebrates with a long fossil record.
They have a skeleton known as the “test” which is made of many plates - each plate is a single crystal of calcite.
They belong to one of two groups: regulars and irregulars. Regular echinoids have radial symmetry and live on the
sea floor. Irregular echinoids are bilaterally symmetrical and may be adapted to burrow in soft sediment.
Range: Late Ordovican - today. The oldest is known from the Late Ordovican
(450 million years ago).
Regular echinoids. Regular echinoids have radial symmetry and live on the sea floor.
Regular sea urchins have




Pentaradial (five-fold) symmetry
10 double columns of plates
5 interambulacral zones
5 ambulacral zones
There are 2 large openings in the test:
1. Peristome – central, circular lower surface
where the mouth opens
2. Periproct – anus at the anus
The lantern
The lantern is a structure that enables the sea
urchin to bite and rasp. It is made up of 50 skeletal
parts worked by 60 muscles.
The tips of the teeth peep out of the mouth, but
most of the structure is tucked inside the sea
urchin.
Irregular echinoids. Irregular echinoids have bilateral symmetrical imposed on five-fold symmetry and are
commonly adapted to burrow.
Sand dollars and relatives




bilateral symmetry
10 double columns of plates (1-5)
5 interambulacral zones (I-V)
5 ambulacral zones
Ambulacral plates are pierced by single or
double pores for the tube-feet (an external
flexible tentacle from the water vascular
system).
All 5 ambulacra are usually identical with very
distinctive petaloid pattern.
There are 2 large openings in the test as in a
regular echinoid
Heart urchins (spatangoids) and their
Periproct:

houses the anal opening and is always
positioned towards the posterior.
Peristome:

The opening through the test associated
with the mouth
Spines and tubercles:


usually rather fine in heart urchins
Spines are long and slender and often
spatulate. (spoon-like). They help with
burrowing action.
ECHINOID EVOLUTION.
Echinoids were slow starters but during the Palaeozoic, echinoids developed the
ability to burrow which enabled them to diversify. The roots of our modern echinoid fauna can be found in the
first cidaroids which appeared in the Permian (about 250 million years ago). Early Jurassic they radiated,
irregular echinoids appeared for the first time and rapidly adapted specializations for deposit feeding, possibly
triggered by ever rising levels of food production.
Evolutionary evidence.
Micraster shows slow, ordered evolutionary changes as the Chalk was slowly
deposited during Cretaceous times.
Useful website: The Echinoid Directory – designed and created by Dr. A.B. Smith
http://www.nhm.ac.uk/research-curation/research/projects/echinoid-directory/