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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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