12 Phylum Echinodermata >> Lesson plan

12  Phylum Echinodermata


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 Echinus acubus.


12.1 Position in animal kingdom


Phylum Echinodermata belongs to the Deuterostomia branch of the animal kingdom and the members of it are enterocoelous coelomates .


a. 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 vertebrates .

Thus, the echinoderms , chordates , and the lesser deuterostome phyla presumably have been derived form a common ancestor.


12.2 Biologic contributions


Among the more striking of  the features shown by the echinoderms are the system of  coelomic channels comprising the water –vascular system, and the metamorphosis from bilateral larva to radial adult .


12.3 Structure and Characteristics


The most noticeable characteristics of the echinoderms are :

  (1) the spiny endoskeleton of plates ;

  (2) the water–vascular system ;

  (3) the pedicellariae   

(4) the dermal branchiae

(5) radial or biradial symmetry .

           a. Body unsegmented ( nonmetameric ) with radial , pentamerous symmetry ;body rounded, cylindric, or star-shaped , with five or more radiating areas , or ambulacra , alternating with interambulacral areas.

b. No head or brain ; few specialized sensory organs ; sensory system of tactile and chemoreceptors , terminal tentacles , photoreceptors , and statocysts .

c. 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 .


Fig. Diagrammatic cross section through the arm of a sea star


d. Endoskeleton of dermal calcareous ossicles with spines or of calcareous spicules in dermis; covered by an epidermis ( ciliated in most ) ; pedicellariae ( in some ) .

e. 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 or  hydropore) usually present .

f. Locomotion  by 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.

g. Digestive system usually complete ; axial or coiled ; anus absent in ophiuroids  .

h. 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 urchin Arbacia


i. 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 cilia

j. respiration by dermal branchiae, by tube feet , by respiratory tree ( holothuroids ) , and by bursae ( ophiuroids ) .

k. Excretory organs absent .

l. 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 .

m. Development through free-swimming , bilateral , larval stages ( some with direct development ) ; metamorphosis to radial adult or subadult form .

n. 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.



Fig. Life cycle of Asterias


               male                   female

                             fertilization of egg

  Young starfish                                 zygote


Metamorphosis                                   2-cell stage


      Brachiolaria                            blastula


bipinnaria                      gastrula  


Typical animal -- Asterias


a. The external feature and construction of Asterias

      disc                       anus

      spines                     oral surface

      aboral surface               madreporite

      ambulacral groove            ray                        

      tube feet                    tentacle                



b. Water-vascular system :

        stone canal             radial canal

        ring canal              connecting canal

        ampulla                podium or tube feet

        lateral canal



    c. perihaemal system

        genital sinus

        ring sinus 

    d. haemal system

        oral haemal ring 

        radial haemal canal

        aboral haemal ring  


Hemal system:


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

                  nerve ring

                  radial nerve

                  nerve plexus 


            hyponeural system 

                  nerve ring

                  radial nerve ( 5 )

            aboral system

                  radial nerve ( 5 )


12.4 Classification


There are abut 6000 living and 20000 extinct or fossil species of Echinodermata .

The 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.


A. Subphylum Pelmatozoa

        Class crinordea

B. Subphylum Eleutherozoa ( all lack a stalk )

        Class Asteroidea

        Class Ophiuroidea

        Class Echinoidea

        Class Holothuroidea



There has been growing recognition that these subphyla were each polyphyletic, and most students of echinoderms  have adopted the following classification .



12.4.1 Phylum Echinodermata


   ( 1 ) Subphylum Asterozoa


   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 .


Fig. Starfishes


b. Class Ophiuroidea

Brittle stars. Typically pentamerous; arms sharply marked off from the disk ; no ambulacral groove.


 (2) Subphylum Echinozoa

Tube feet restricted to the compact body; never stalked ; radial symmetry , often with superimposed  bilateral appearance


    a. Class Echinoidea

Sea urchins. Pentamerous, without arms or free rays ; test of calcareous plates bearing movable spines .

Strongly locentrotus , Arbacia


    b. Class Holothuroidea

Cucumber-shaped echinoderms with no arms ; spines absent ; microscopic ossicles  embedded in thick muscular wall ; anus present ; ambulacral grooves closed ; tube feet with suckers ; circumoral tentacles ( modified tube feet ) : pedicellariae absent ; madreporite plate internal .


Fig. External feature of Holothuroids

Fig. Internal structure of Holothuroids


 (3) Subphylum Crinozoa


  Several Class of radial symmetrical attached forms ; one surviving class.


Class crinoidea


     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. Antedon

Fig. metaerinus

Fig. A feather star from the Red Sea perched on a coralline rock


(a) A stalked crinoid ( or sea lily) with five arms.

(b) Oral structure of a crinoid crown, including a section of one arm.



Review Questions


1. What constellation of characteristics is possessed by echinoderms and is found in no other phylum?

2. Distinguish the following groups of echinoderms from each other: Crinoidea, Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea.

3. What structure are involved in the following functions in sea stars? Briefly describe the action of each: respiration, feeding and digestion, excretion, reproduction.

4. Briefly describe development in sea stars, including metamorphosis.

5. Define the following: pedicellariae, madreporite, respiratory tree, Aristole’s lantern, water vascular system, tube foot, protostome and deuterostome.