PHYLUM ECHINODERMATA
Echinoderms are marine animals such as sea stars, brittle stars, sea urchins, sea cucumbers and sea lilies. They are an
ancient group of animals extending back to at least the Cambrian period. They occur in all oceans at all depths. No
parasitic echinoderms are known, but some are commensal.
Echinoderms have a combination of characteristics found in no other phylum:
1) An endoskeleton of large plates or small scattered ossicles
2) A water-vascular system
3) Pedicellariae
4) Dermal branchiae
5) Basic pentaradial symmetry in adults
Characteristics of Phylum Echinodermata:
Unique water-vascular system of coelomic origin
Free-living
Unsegmented body
Pentaradial symmetry
Body is round, cylindrical or star shaped, with 5 or more radiating areas, or ambulacra
No head
Triploblastic
Extensive coelom
Endoskeleton of dermal calcareous ossicles with spines or of calcareous spicules in dermis, covered by epidermis
Complete digestive system
Skeletal elements connected by ligaments of mutable collagenous tissue under neural control
Locomotion by tube feet which project from ambulacral areas, movement of spines or movement of arms
Nervous system with circumoral ring and radial nerves
No brain
, Sensory system of tactile and chemoreceptors, podia, terminal tentacles, photoreceptors and statocysts
Autotomy and regeneration of lost parts
Some reproduce asexually by fragmentation
Sexes separate with large gonads
External fertilisation
Development through free-swimming, bilateral, larval stages
Metamorphosis to radial adult
Radial cleavage and regulative development
No excretory organs
Respiration by papulae, tube feet, respiratory tree and bursae
Blood-vascular system (hemal system) reduced and plays a minor role in circulation. Circulation by peritoneal cilia
CLASS ASTEROIDEA:
This class includes sea stars. They are common along the shoreline and also live on muddy or sandy substrates and
coral reefs. They are often brightly coloured and range in size.
Form and Function:
External features:
Composed of a central disc that
merges with tapering arms
The body is flattened, flexible and
covered with a ciliated, pigmented
epidermis
Their mouth is on the underside,
surrounded by a soft peristomial
membrane
An ambulacrum runs from the mouth on the oral side to the tip of each arm
An ambulacral groove is found along the middle of each ambulacrum, bordered by rows of tube feet, protected
by movable spines. A large radial nerve is in the centre of each ambulacral groove covered only by thin
epidermis
The aboral surface is rough and spiny. Around the bases of spines are groups of minute pincerlike pedicellariae,
bearing tiny jaws manipulated by muscles. These jaws keep the body surface free of debris and aid in food
capture
Papulae are soft, delicate projections of the coelomic cavity, covered only with epidermis and lined internally
with peritoneum. They extend through spaces between ossicles and are involved with respiration.
Also, on the aboral side are an inconspicuous anus and madreporite, a calcareous sieve plate leading to the
water-vascular system.
Endoskeleton:
Beneath the epidermis is a mesodermal endoskeleton of small calcareous plates/ossicles, bound together with
connective tissue. This connective tissue is mutable collagen, called catch collagen, that is under neurological
control. Catch collagen can be changed from a liquid to a solid quickly when stimulated by the nervous system.
Ossicles are penetrated by a meshwork of spaces, filled with fibres and dermal cells. This network is called a
stereo
Muscles in the body wall move the rays and can partially close the ambulacral grooves by drawing their
margins together
Coelom, Excretion and Respiration:
Coelomic compartments of larval echinoderms give rise to many structures as adults
Spacious body coelom filled with fluid containing amoebocytes, bathes internal organs and projects into
papulae
The ciliated peritoneal lining of the coelom circulates the fluid
Echinoderms are marine animals such as sea stars, brittle stars, sea urchins, sea cucumbers and sea lilies. They are an
ancient group of animals extending back to at least the Cambrian period. They occur in all oceans at all depths. No
parasitic echinoderms are known, but some are commensal.
Echinoderms have a combination of characteristics found in no other phylum:
1) An endoskeleton of large plates or small scattered ossicles
2) A water-vascular system
3) Pedicellariae
4) Dermal branchiae
5) Basic pentaradial symmetry in adults
Characteristics of Phylum Echinodermata:
Unique water-vascular system of coelomic origin
Free-living
Unsegmented body
Pentaradial symmetry
Body is round, cylindrical or star shaped, with 5 or more radiating areas, or ambulacra
No head
Triploblastic
Extensive coelom
Endoskeleton of dermal calcareous ossicles with spines or of calcareous spicules in dermis, covered by epidermis
Complete digestive system
Skeletal elements connected by ligaments of mutable collagenous tissue under neural control
Locomotion by tube feet which project from ambulacral areas, movement of spines or movement of arms
Nervous system with circumoral ring and radial nerves
No brain
, Sensory system of tactile and chemoreceptors, podia, terminal tentacles, photoreceptors and statocysts
Autotomy and regeneration of lost parts
Some reproduce asexually by fragmentation
Sexes separate with large gonads
External fertilisation
Development through free-swimming, bilateral, larval stages
Metamorphosis to radial adult
Radial cleavage and regulative development
No excretory organs
Respiration by papulae, tube feet, respiratory tree and bursae
Blood-vascular system (hemal system) reduced and plays a minor role in circulation. Circulation by peritoneal cilia
CLASS ASTEROIDEA:
This class includes sea stars. They are common along the shoreline and also live on muddy or sandy substrates and
coral reefs. They are often brightly coloured and range in size.
Form and Function:
External features:
Composed of a central disc that
merges with tapering arms
The body is flattened, flexible and
covered with a ciliated, pigmented
epidermis
Their mouth is on the underside,
surrounded by a soft peristomial
membrane
An ambulacrum runs from the mouth on the oral side to the tip of each arm
An ambulacral groove is found along the middle of each ambulacrum, bordered by rows of tube feet, protected
by movable spines. A large radial nerve is in the centre of each ambulacral groove covered only by thin
epidermis
The aboral surface is rough and spiny. Around the bases of spines are groups of minute pincerlike pedicellariae,
bearing tiny jaws manipulated by muscles. These jaws keep the body surface free of debris and aid in food
capture
Papulae are soft, delicate projections of the coelomic cavity, covered only with epidermis and lined internally
with peritoneum. They extend through spaces between ossicles and are involved with respiration.
Also, on the aboral side are an inconspicuous anus and madreporite, a calcareous sieve plate leading to the
water-vascular system.
Endoskeleton:
Beneath the epidermis is a mesodermal endoskeleton of small calcareous plates/ossicles, bound together with
connective tissue. This connective tissue is mutable collagen, called catch collagen, that is under neurological
control. Catch collagen can be changed from a liquid to a solid quickly when stimulated by the nervous system.
Ossicles are penetrated by a meshwork of spaces, filled with fibres and dermal cells. This network is called a
stereo
Muscles in the body wall move the rays and can partially close the ambulacral grooves by drawing their
margins together
Coelom, Excretion and Respiration:
Coelomic compartments of larval echinoderms give rise to many structures as adults
Spacious body coelom filled with fluid containing amoebocytes, bathes internal organs and projects into
papulae
The ciliated peritoneal lining of the coelom circulates the fluid
