1. SUPERPHYLUM RADIATA
The radiata isa superphylumwhichincludesboth the echinoderms,cnidariansand ctenophores
and is not part of the usual classification system and is not used by the biologists. The group includes
radially symmetric animals of Eumetazoa1
. The echinoderms, however, are members of the Bilateria,
because theyexhibitbilateral symmetryintheirdeveloping stages. Their radial symmetry is secondary.
In 1983 Thomasdefinedasub-kingdomcalledRadiataConsistingof the phylaporifera,myxozoa,
placozoa, cnidaria and ctenophore (that is, all the animals that are not in bilateria). The five kingdom
classification of Lynn Margulis and K. V. Schwartz keeps only Cnidaria and Ctenophora in Radiata.
We will deal hear with the Cnidaria and Ctenophora with respect to their general characters.
COELENTRATA (OR CNIDARIA)
The cnidarians2
are an aquaticgroup of organisms, commonly known as hydroids, jellyfish, sea
anemone andcorals.Theyare tentacle bearing, radial or biradial animals with sac-like body composed
of two basic cell layers. The inner layer of cells, called the gastrodermis, lines the hollow space, the
coelenterons, which functions in digestion and transport and into which there is single opening, the
mouth.The outercell layerconsistsof an epidermis.The gastrodermisandepidermisare separatedby a
thin,acellularmatrix or mesolamella or by a jelly-like material containing cells called the mesoglea or
mesenchyme.
All Cnidaria are constructed from eight basic cell types – epithelial, muscular, nervous,
glandular, reproductive, interstitial, mesenchyme and cnidoblasts. Cnidoblasts are most frequently
presentontentaclesandcontainstingingstructures –the nematocystswhichfunctionin defence and in
the capture of food; all cnidarians are carnivorous in habit.
All cnidarianshave tissue levelof organizationhaving,epiderma,nervous,digestive and
musculartissues,butlackingspecializedorgansystem.Fundamentallytwobodyformsare repre:Polyp
and Medusa(we will talkaboutthese later)
EXTERNAL MORPHOLOGY
Coelenteratesare aacoelomate andradiallysymmetrical lowerinvertebrates(Radiata). The
coelenteratesare representedbytwomorphologicallydifferenttypesof individuals, polypsand
medusa.
Polypsare sessile withatubularbody(eg. Hydra,Metridium (Seaanemone))
Medusaare free swimmingumbrellaorbell-shapedbody.
Some coelenteratespassthroughbothstagesintheirlife cycle withanalternationof generations.
1 Eumetazoa is a clade which includes all major animal phylaexceptsponges.
2 Cnidos, thread; Gk

2. INTERNAL STRUCTURE
Internal structure of Cnidarianscanbe understoodbystudyingthe histologyof Hydra.
GASTROVASCULARCAVITY
Almostall cnidarianshave acentral cavitycalled coelenterons,functionallyreferredtoasgastrovascular
cavity.It has onlyone opening –the mouth.There isno anusand no excretorypore.The undigested
foodand wastesexitthe cavitythroughthe mouthopening.
BODY WALL
There body wall iscomposedof twolayersof cells,outerlayeriscalledepidermisandthe innerlayeris
calledgastrodermis/endodermis.The twolayersare separatedfromeachotherbya thindelicate,non-
cellulargelatinouslayercalledmesogleaorsupportinglamella.
1. EPIDERMIS
The epidermisiscomposedof small,more orlesscuboidal cell.Itisaprotective andsensory
layer.The epidermisiscoveredexternallybyathincoatingof cuticle.Ithasvarioustypesof cells
as follows.
a. Epithelio-muscle cells

3. Most of epidermisiscomposedof epithelio-musclecellsthathelpsinbothmuscular
contractionand epithelialcovering.The epithelial partextendsuptobodysurface and the
basal muscularpart is drawnout intotwomuscle processesalongthe longitudinal axis.
Muscle processcontainscontractile fibril,the myoneme.
b. Glandcells
These are tall cellsfoundchieflyonpedal discandaroundmouthregion.These secretea
mucus-like stickymaterial whichservesforattachmentandprotection.
c. Interstitial cells
Theyare presentinthe interstitial spacesbetweenepithelio-muscle cells.Theyare small
roundundifferentiatedcellsandare capable of developinganyotherkindof cells,suchas
reproductive,glandular,stingingetc.
d. Cnidoblasts
Many of the interstitialcellsof epidermisdevelopintospecializedcellscalled cnidoblasts.
These are speciallypresentonthe tentaclesarrangedinclustersorbatteries.Theyare oval
shapedcellswithbasal nucleusandpossessasac-likeorganoid –the nematocyst.Itis like a
capsule enclosingacoiledtube orthread.
e. Sensorycells
Theyoccur scatteredthroughoutthe epidermisamongepithelio-musclecellsandare
speciallyfoundontentacles,hypostomeandpedal disc.
f. Nerve cells
Throughoutthe evolutionarytrend,true nerve cellsorganglioncell occursforfirsttime in
coelenterates.Theyare derivedfrominterstitial cellsof epidermis.Theyoccurat the base of
epithelio-muscle cellsjustabove theirmuscle processes..
g. Germcells
Duringsummer,interstitial cellsof some regionsof body,dividerepeatedlyandproliferate
like reproductivecellsforminggonads,whichlaterdifferentiate intotestesandovaries.
2. GASTRODERMIS
The coelenteronsisinternallylinedbygastrodermis.Itisformedchieflyof large,columnar cells.
Thislayerismainlynutritive infunction.The gastrodermisiscomposedof followingtypesof
cells.
a. Endothelio-muscle ornutritive cells
These are most abundantcellsformingbulkof gastrodermis. Theseresemblethe epithelio-
muscle cellsof epidermisexceptthattheirbasal contractile processesare single,more
delicate andorientedatrightanglestothe processesof the epithelio-musclecells.There
contractionreducesthe diameterof the bodyandtentacles,whichbecomesnarrowerand
longer.
The free endmay beartwo whip-likeflagellaandbluntpseudopodialike projections.The
movementof flagellakeepsthe foodinthe coelenteronsinmotion.The pseudopodialike
projectionscanengulf foodparticles.
b. Endothelial-glandcells

4. Theyare smallerthannutritive cellsandoccursscatteredamongnutritive cells.Theylack
muscle tailsatbasal end,butmay bearone or two flagella.The glandcellsare absentinthe
tentacles.Theyare of twotypes.
 Enzymaticglandcells:secrete digestive enzymesintogastrovascularcavityfor
extracellulardigestion.
 Mucous glandcells:secrete aslimyfluidservingaslubricants.
c. Interstitial cells
A fewinterstitial cellsoccurbetweenthe basesof nutritivecells.These are totipotentcells
and thuscan transform intoall other type of cells.
d. Sensorycells
Sensorycellsare foundinbetween nutritive cells.Theyare supposedtobe stimulatedon
entryof preyintogastrovascularcavity.
e. Nerve cells
These are similartothose presentinepidermis,exceptthattheyare presenthere infewer
no.than epidermis.
3. MESOGLEA
It isnon-cellularthinlayersecretedbybothepidermis andendodermisandseparatesthe
epidermisandgastrodermis.Itservesforattachmentof cellularlayers.
4. CNIDARIAN LIFE FORMS
The cnidariansare modifiedintotwobasiclife forms –polypandmedusa.Polypisusuallya
sedentaryform,adaptedtononmotile existence anddominantinclasshydrozoaandanthozoa.
 the polypsare cylindrical andone endremainsattachedtoa substratumwiththe help
of pedal disc(Proximalend).The distalendisprovidedwithtentaclesandanopening
calledhypostome whichopensintothe central cavity –the coelenteronor
gastrovascularcavity.The whole bodysurface exceptthe pedal dischascnidocytes.
 The medusaare free swimmingformsandgive rise toa group called – jellyfish.Bodyis
umbrellalike consistingof subumbrellarandexumbrellarsurface.A projectioncalled
manubriumhangsfromsubumbrellarsurface bearingamouthat free end.The mouth
leadsintogastrovascularcavity.The edge of bell bearsnumeroustentaclespossessing
cnidocytes.
5. FEEDING
Cnidarianscandigestproteins,fats,some carbohydratesbutnotstarch.Small annelids,
crustaceans,insectlarvae are chief foodof cnidarians.The cnidocytesdischargetoxinintothe
bodyof preyand paralyze itwhichisgraduallydrawntowardsthe mouthwithhelpof tentacles.
Proteolyticenzymesare secretedinthe coelenteronsbyglandcellsforextracellulardigestion.
The semidigestedfood are engulfedbysome endodermal celsandrestof digestionoccur
intracellularlyanddigestedfoodistransferredtootherpartsof body.Undigestedfoodis
ejectedthroughmouth.
6. RESPIRATION AND EXCRETION

5. Cnidarianshave noorganfor respirationandexcretion.Asthe bodywall isthinandthe
epidermisaswell asthe gastrodermisare exposedtowater.The exchange of gasesandexcetion
of nitrogenouswaste occursthroughsimplediffusion.
7. REPRODUCTION
The reproductionoccursboth bysexual andasexual means
 Asexual reproduction:occursduringsummerwhenfoodsupplyisabundant
It occurs by budding, andfission(transverseandlongitudinalinpolyps)
 Sexual reproduction:mostcnidariansare dioeciousbutsome are monoeciuos
(hermaphrodite).Itusuallyoccursinwinters.Sexual fertilizationisalwayscross
fertilization.
 POLYMORPHISM IN COELENTERATA
 The occurrence of more than one type of structurally and functionally different
individuals within a population is called polymorphism. The class Hydrozoa of phylum
Coelenterata includes a large number of colonial species that contain more than one
form of individuals which are called zooids
 Coelenterates have two basic zooids, polyp and medusa. All other types of
zooids are modifications of these two types of zooids.
 Polyp has a tubular body with a mouth surrounded by tentacles at one end. Other
end is blind and usually attached by a pedal disc to the substratum. Polyps are
typically sessile.
 Polyps are concerned with feeding, protection and asexual reproduction.
 Medusa has a bowl or umbrella shaped body with marginal tentacles and mouth is
centrally located in a projection called manubrium on the ventral concave surface.
Medusae are generally motile and concerned with sexual reproduction.
 Polyp and medusa are in fact homologous structures and from one another can
easily be derived.

6. Figure 1: Polymorphism in Cnidaria: example Aurelia
 Modifications of Polyp
 Gastrozooids or feeding zooids are typical polyps with a mouth and
surrounding tentacles.
 Dactylozooids which are used for defence are polyps without mouth and
usually with a long basal tentacle.
 Gonozooids are reproductive zooids derived from polyp, which produce sexual
medusae or gonophores.
 Modifications of Medusa
 Nectophore or nectocalyx or swimming bell is a medusa modified for sexual
reproduction.
 Pneumatophore or float is a bladder-like modified medusa filled with mixture of
gases and helping the colony to float on the surface.

7.  Phyllozooid or bract is leaf-like zooid, studded with nematocysts and serving to
protect the colony.
 TYPES OF POLYMORPHISM
 A few coelenterates, such as Hydra and sea anemone
are monomorphic in which only polyp stage is found but other coelenterates
exhibit polymorphism.
 Dimorphism: Many hydrozoan colonies like Tubularia and Campanularia have
only two types of zooids, the feeding zooids
or gastrozooids and medusae or nectophores that bud off from the stem or
gastrozooids. These are dimorphic colonies in which blastostyles are not present.
 Trimorphism: Some species like Obelia and Plumularia are trimorphic because
besides gastrozooidsand medusa, they also have medusa-
producing gonozooids or blastostyle.
 Polymorphism: Coelenterates having more than three types of individuals are
called polymorphic, e.g. Hydractinia which has five types of zooids, each performing a
specialized function. Gastrozooidsare for feeding, spiral dactylozooids for protection,
long tentaculozooids have sensory function. Skeletozooids are spiny projections of
chitin for protection and gonozooids are reproductive zooids that produce male or
female medusae.
 Highly modified polymorphism: In order Siphonophora, such as Diphyes,
Halistemmia, Stephaliaand Physalia, zooids are so much modified that they appear
like organs of a single body rather than individuals of a colony. They are mostly
pelagic in habit. In Physalia, zooids are in units called cormidia, which
bear gastrozooids, small and large dactylozooids with long and short tentacles and
branched gonozooid with gonophores. In Diphyes colonies are linear with one or
more nectophoreslocated at the apical end. Cormidia are also repeated in a linear
succession. In Velella and Porpita, there is a single large central gastrozooid with a
mouth, around which are arranged concentric rows of gonozooids and dactylozooids.
The whole colony looks like a single individual.
 ORIGIN OF POLYMORPHISM
 There are two theories to explain the origin of polymorphism in coelenterates.

8.  Polyorgan theory: This theory was proposed by Huxley (1859), Eschscholtz
(1829), E. Metschnikoff (1874) and Muller (1871), according to which individuals of a
colony are actually organs of a medusoid individual, which have multiplied and
migrated from their primitive positions to the current evolved positions.
 Polyperson theory: This theory was first proposed by Leuckart (1851), Vogt
(1848), Gegenbaur (1854), Kolliker (1853), Claus (1863) and later strongly supported
by E. Haeckel (1888), Balfour (1885) and Sedgewick (1888). According to this theory
colony is not a single individual but various parts of the colony are modified individuals
which have changed their structure due to division of labour. They have all modified
from the primitive zooid which was a polyp.
 Medusome theory: This theory was proposed by Haeckel (1888) as a
compromise between the above theories. The theory says that the siphonophore larva
formed from gastrula was a medusoid individual, from which zooids or persons
appeared by budding from the subumbrella.
 SIGNIFICANCE OF POLYMORPHISM
 The phenomenon of polymorphism is essentially one of division of labour in
which specific functions are assigned to different individuals. Thus, polyps are
modified for feeding, protection and asexual reproduction, while medusae are
concerned with sexual reproduction. This distribution of functions among diversified
individuals and their subsequent modifications in coelenterates may have resulted
from their initial simple organization and lack of organ specialization. Polymorphism
gave the colonies competitive edge in protection and food gathering and eventual
survival.