Phylum Echinodermata
Sea Star, Sea Urchin, Sea Cucumber
General Characteristics
• Spiny skinned
• pentaradial symmetry = as adults have five
similar parts arranged in circle around a
central point
• exclusively marine, adapted to a bottomdwelling existence
• examples include sea stars (‘starfish’), sea
urchins, sand dollars, sea cucumbers
• most species reproduce sexually, have separate sexes, and
have external fertilization. Note that the larvae of echinoderms
are ciliated and bilaterally symmetric .
• Many echinoderms are recognized by their pentaradial (fivefold) symmetry. It is important to understand that the radial
symmetry of echinoderms is superficial and almost certainly
secondarily derived. This means that the ancestor of
echinoderms (the ancestor it shares in common with all other
bilaterians) was likely bilaterally symmetric. If we look carefully
at a typical echinoderm like the sea star we will note that the
five part organization can be divided into mirror-image right
and left halves - the hallmark of bilateral symmetry.
• An easy way to see this is to find the madreporite and imagine
a line connecting it to the tip of the arm opposite the
madreporite. This line will divide the animal into right and left
halves.
Muscular/ Skeletal System
• Mouth on the bottom
side of organism (oral
surface). Other side=
aboral surface
• have an internal
skeleton
(endoskeleton)
composed of
interlocking calcium
carbonate plates
Water Vascular System
• Contains a water vascular system
• Water enters body through a sieve plate
(=madreporite) The water is transport through
the ring canal and the radial canal to power the
tube feet
• have tube feet, which are versatile structures
used for locomotion, gas exchange, feeding,
attachment, and sensory reception!
• Tube feet:..\..\..\Lesson Videos\Biology
11\Arthropods and Echinoderms\Starfish or Sea
Star (Asteroidea).flv
Digestion
• Use tube feet to open
bivalves, to capture preys
• Sea stars and sea urchins
contain pedicellaria
= grasping appendages
(food gathering, cleaning, & defense against
predators)
• Sea stars can flip its stomach out of the mouth and
digest/ eat the food outside its body. It will flip its
stomach back in when finished.
• Herbivores (e.g. sea urchins) have 5-part jaws
to scrape algae
• Some are filter feeders (sea lillies, brittle stars)
• Some detritus feeders
Respiration
• Diffusion through tube feet
Circulatory
• Done by digestive gland and fluid in body
(located in pseudocoelem)
Excretory
• Solid food waste anus
• Nitrogen waste tube feet, gill, all over body
Nervous System
• No brain (radial symmetry)
• Nerve rings around mouth; eyespot; statocysts
Movement
• Use tube feet
Reproduction
• Echinoderm sexes are generally separate
• Fertilization and development of young are
external, larval stage in life cycle
Class Asteroida (Sea Stars)
• 5-26 arms
• eyespots located on tip of each arm
• push stomachs out of body through mouth to
devour prey
• can regenerate lost or damaged arm, but is
very slow
• gills protrude through skin
Crown of thorns starfish
• Reading and species file
• Video:..\..\..\Lesson Videos\Biology
11\Arthropods and Echinoderms\Crown-ofThorns starfish.flv
Know your anatomy
Worksheet time