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Class Echinoidea

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Echinoids are free moving echinoderms

They are commonly known as sea urchins, hearturchins and sand dollars.

There are approximately 940 species of

echinoids.

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More or less globular or disc shaped,

with no arms;

compact skeleton or test with closely fitting

plates;

movable spines;

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ambulacral grooves closed;

tube feet with suckers;

Pedicellariae present

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REGULAR ECHINOIDS (Sea urchins)

Body is more o r less spherical in shape.

Armed with relatively long movable spines.

Colored brown, black, purple, green, white and red. Few are multicolored (Diadema antillarum)

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Diadema antillarum

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Sea urchin calcareous shell: aboral and oral view

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IRREGULAR ECHINOIDS (heart urchins and

sand dollars)

All are adapted for burrowing in sand

Have much smaller and more numerous spines

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Heart urchins (spatangoids) are more or less

oval in shape

Oral surface is flattened

Aboral surface is convex

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Podia are degenerate or absent around the

circumference of the body.

Have certain specialized spines ( Clavules)

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Sand dollars (clypeastroids) body are greatly

flattened.

Displays a circular circumference

Periproct is venral and ocated in the posterior

interambulacrum.

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Keyhole sand dollars contain elongated notches

(opening known as lunules)

Aboral surface bears conspicuous petaloids

No phyllodes

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Spheridia and spination are similar to heart

urchin

No clavalus

Poison pedicellariae are present.

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Flagellated epidermis covers the outer surface,

including spines

beneath has nervous layer and connective tissue

dermis that contains the skeleton

Plates are arrange in rows (oral and aboral)

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Close up of the Madreporite of an sea star

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Spines and podia are used for movement

Has burrowing behavior

Irregular echinoids are adapted for life of

burrowing in sand.

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Sea urchins feed on all types of organic material,

plant or animal, living or dead.

Sea urchins living in a great depths are detritus

feeders.

Have highly developed chewing apparatus Aristotle lantern

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A large principal coelom

Number of minor sub compartments

Coelomic fluids is the principal circulatoryradium

A hemal system is present

In regular echinoids the five parts of peristomialgills is for gas exchange

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In heart urchins, nerve ring is located at the

peristome.

The numerous sensory cells in the epithelium,

particularly on the spines, pedicellariae and

podia, composed the major part.

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Dioecious

Display no sexual dimorphism

Fertilization takes place in sea water

Brooding

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Unfertilized egg

Fertilized egg

Embryo

LarvalMetamorphosis

Juvenile

Adult

Life Cycle Stages of S. purpuratus

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