Sea urchins, the “pin cushions
” of the sea , are bottom dwellers that attach themselves by
hydraulically operated tube feet and feed by scraping the rocks with a peculiar
five-toothed mechanism called “ Aristotle’s lantern ”. This is a group of
12.1 Position in
Phylum Echinodermata belongs to the
Deuterostomia branch of the animal kingdom and the members of it are
enterocoelous coelomates .
Primitively, deuterostomes have the following embryologic features in common ;
b. anus developing from or near the
blastopore , and mouth developing else where ;
c. coelom budded off from the archenteron (
enterocoel ) ;
d. radial and indeterminate cleavage ;
e. endomesoderm ( mesoderm derived form or
with the endoderm ) from enterocoelic pouches ;
f. though only distantly related , the
Echinodermata is the only major invertebrate group showing affinities with the
the echinoderms , chordates , and the lesser
deuterostome phyla presumably have been derived form a common ancestor.
Among the more striking ofthe features shown by the
echinoderms are the system ofcoelomic channels comprising the water –vascular system, and the
metamorphosis from bilateral larva to radial adult .
12.3 Structure and
The most noticeable characteristics of the
echinoderms are :
(1) the spiny
endoskeleton of plates ;
water–vascular system ;
(3) the pedicellariae
(4) the dermal branchiae
(5) radial or biradial symmetry .
unsegmented ( nonmetameric ) with radial , pentamerous
symmetry ;body rounded, cylindric, or star-shaped , with five or more radiating
areas , or ambulacra , alternating with interambulacral areas.
No head or brain ; few specialized sensory organs ;
sensory system of tactile and chemoreceptors , terminal tentacles ,
photoreceptors , and statocysts .
Nervous system with circumoral ring and radial nerves; usually two or three
systems of networks located at different levels in the body, varying in degree
of development according to group .
Diagrammatic cross section through the arm of a sea star
Endoskeleton of dermal calcareous ossicles with spines or of calcareous
spicules in dermis; covered by an epidermis ( ciliated
in most ) ; pedicellariae ( in some ) .
A unique water–vascular system of coelomic origin that extends from the body
surface as a series of tentacle-like projections ( podia , or tube feet ) that
are protracted by increase of fluid pressure within them ; an opening to the
exterior ( madreporite orhydropore) usually present .
Locomotionby tube feet , which
project from the ambulacral areas , or by movement of spines , or by movement
of arms , which project from central disc of body.
Digestive system usually complete ; axial or coiled ;
anus absent in ophiuroids.
Coelom extensive , forming the perivisceral cavity and
the cavity of the water-vascular system ; coelom of enterocoelous type ;
coelomic fluid with amebocytes .
Fig. Internal structure of the sea
blood-vascular system ( hemal system ) much reduced , playing little, if any ,
role in circulation , and surrounded by extensions of coelom ( perihemal
sinuses ) , main circulation of body fluids ( coelomic fluids ) by peritoneal
j. respiration by dermal branchiae, by tube
feet , by respiratory tree ( holothuroids ) , and by bursae ( ophiuroids ) .
Excretory organs absent .
Sexes separate ( except a few hermaphroditic ) with large gonads , single in
holothuroids but multiple in most ; simple ducts , with no elaborate copulatory
apparatus or secondary sexual structures ; fertilization usually external; eggs
brooded in some .
Development through free-swimming , bilateral , larval
stages ( some with direct development ) ; metamorphosis to radial adult or
subadult form .
Autotomy and regeneration of lost parts conspicuous .
Fig.Comparison of different larval types of echinoderms.Arrows indicate possible evolutionary
relationship to a hyporthetical ancestral form.
Life cycle of Asterias
fertilization of egg
Typical animal -- Asterias
The external feature and construction of Asterias
Water-vascular system :
stone canalradial canal
ring canalconnecting canal
or tube feet
oral haemal ring
radial haemal canal
aboral haemal ring
The so-called hemal system is not very
well developed in asteroids, and its function in all echinoderms is unclear.
The hemal system has little or nothing to do with circulation of body fluids.
It is a system of tissue strands enclosing unlined sinuses and is itself
enclosed in another coelomic compartment, the perihemal channels. The hemal
system may be useful in distributing digested products, but its specific
functions are not really known.
e. nervous system
oral or ectoneural system
radial nerve ( 5 )
radial nerve ( 5 )
are abut 6000 living and 20000 extinct or fossil species of Echinodermata
traditional classification of the echinoderms placed all the free-moving forms
oriented with oral side down in the subphylum Eleutherozoa, containing most of
the living species. The other subphylum, Pelmatozoa, contained mostly forms
with stems and oral side up; most of the extinct classes and the living
crinoidea) belong to this group.
Subphylum Eleutherozoa ( all lack a stalk )
There has been growing recognition that
these subphyla were each polyphyletic, and most students of echinodermshave adopted the following
12.4.1 Phylum Echinodermata
( 1 ) Subphylum
With radial symmetry; never
stalked; movable arms .
a. Class Asteroidea
Starfishes. Typically pentamerous ; arms usually not sharply marked off from the
disk ; ambulacral groove present .
b. Class Ophiuroidea
Brittle stars. Typically pentamerous;
arms sharply marked off from the disk ; no ambulacral
(2) Subphylum Echinozoa
feet restricted to the compact body; never stalked ;
radial symmetry , often with superimposedbilateral appearance
a. Class Echinoidea
Sea urchins. Pentamerous, without
arms or free rays ; test of calcareous plates bearing
movable spines .
locentrotus , Arbacia
b. Class Holothuroidea
echinoderms with no arms ; spines absent ; microscopic ossiclesembedded in thick muscular wall ; anus
present ; ambulacral grooves closed ; tube feet with suckers ; circumoral
tentacles ( modified tube feet ) : pedicellariae absent ; madreporite plate
External feature of Holothuroids
Internal structure of Holothuroids
(3) Subphylum Crinozoa
Several Class of radial symmetrical
attached forms ; one surviving class.
Sea lilies and feather
stars . Aboral attachment stalk of dermal ossicles;
mouth and anus on oral surface , five arms branching at base and bearing
pinnules; ciliated ambulacral grooves on oral surface with tentacle-like tube
feet for food-gathering ; spines , madreporite, and pedicellariae absent .
Fig. A feather star from the Red Sea
perched on a coralline rock
(a) A stalked crinoid ( or sea lily) with five
(b) Oral structure of a crinoid crown, including a section of one arm.
What constellation of characteristics is possessed by echinoderms and is found
in no other phylum?
Distinguish the following groups of echinoderms from each other: Crinoidea,
Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea.
What structure are involved in the following functions
in sea stars? Briefly describe the action of each: respiration, feeding and
digestion, excretion, reproduction.
Briefly describe development in sea stars, including metamorphosis.
Define the following: pedicellariae, madreporite, respiratory tree, Aristole’s
lantern, water vascular system, tube foot, protostome and deuterostome.