5 The radiate animals - Phylum Coelenterata >> Lesson plan

5  The radiate animals — Phylum Coelenterata ( Cnidaria )


The Phylum Coelenterata includes the polyps, jellyfishes, sea anemones, and corals. All of these animals have a body wall consisting of two layers of cells, between which is a jellylike substance, the mesoglea. Within the body is a single gastrovascular cavity, or coelenteron. Because of the presence of two cellular layers, Coelenterates are side to have a tissue-level organization. They are also acoelomates; that is, they don’t possess a second body cavity, the coelom .


5. 1 Position in animal kingdom


Phylum Cnidaria are characterized by primary radial or biradial symmetry . Radial symmetry , in which the body parts are arranged concentrically around the oral-aboral axis , is particularly suitable for sessile or sedentary animals .

The phylum has not advanced generally beyond the tissue level of organization, although a few organs occur .  


5.2 Biologic contributions


a. The phylum has developed two well-defined germ layers , ectoderm and endoderm ; a third , or mesodermal ,layer , which is derived embryologically from the ectoderm , is present in some . The body plan is saclike , and the body wall is composed of two distinct layers , epidermis and gastrodermis , derived from the ectoderm and endoderm , respectively . The gelatinous matrix , mesoglea , between these layers may be structureless , may contain a few cells and fibers , or may be composed largely of mesodermal connective tissue and muscle fibers .

b. An internal body cavity, the gastrovascular cavity , is lined by the gastrodermis and has a single opening , the mouth , which also serves as  the anus .

c. Extracellular digestion occurs in the gastrovascular cavity , as does intracellular digestion in the gastrodermal cells .


Fig.Hydra capturing a water flea, having already swallowed another


d. Most radiates have tentacles , or extensible projections around the oral end , that aid in food capture .

e. The first true nerve cells ( protoneurons ) occur in the radiates , but the nerves are arranged as a nerve net , with no central nervous system .

f. Sense organs appear first in the radiates and include well-developed statocysts (organs of equilibrium ) and ocelli.


Fig. The nerves are arranged as a nerve net


g. Locomotion in the free-moving forms is achieved either by muscular contractions . However the groups are still better adapted to floating or being carried by currents than to strong swimming . 

h. Polymorphism  (polyp stage and medusa stage ) in the cnidarians has widened their ecologic possibilities .

i. Some unique features are found in this phylum , such as nematocysts ( stinging  organoids ) in Cnidarians .


5.3 characteristics


a. Entirely aquatic , some in fresh water but mostly marine

b. Radial symmetry or biradial symmetry around a longitudinal axis with oral and aboral ends ; no definite head

c. Two basic types of individuals: polyps and medusae

d. Exoskeleton or endoskeleton of chitinous, calcareous , or protein components in some

g. Special stinging cell organoids called nematocysts in either or both epidermis and gastrodermis ; nematocysts abundant on tentacles , where they may form batteries or rings

h. Nerve net with symmetric and asymmetric synapses; with some sensory organs; diffuse conduction


Fig. nematocysts


e. Body with two layers, epidermis and gastrodermis, with mesoglea ( diploblastic ) ; mesoglea with cells and connective tissue ( ectomesoderm ) in some ( triploblastic )

f. Gastrovascular cavity (often branched or divided with septa ) with a single opening that serves as both mouth and anus; extensible tentacles usually encircling the mouth or oral region

i. Reproduction by asexual budding  ( in polyps ) or sexual reproduction by gametes ( in all medusae and some polyps ) ; sexual forms monoecious or dioecious ; planula larva ; holoblastic cleavage .   

j. No excretory or respiratory systems

k. No coelomic cavity


5.4 Classification


a. Class Hydrozoa

This class includes the freshwater polyps , the small jellyfishes , the hydroid zoophytes, and a few stony corals .

b. Class Scyphozoa

Most of the large jellyfishes are placed in this class .

c. Class Anthozoa

   In this class are included the sea anemones , and most of the stony and horny corals . 


5.4.1 Class Hydrozoa


(1) Characteristics


   a. solitary or colonial ;

   b. asexual polyps and sexual medusae , although one type may be suppressed ;

   c. hydranths with no mesenteries.

   d. medusae ( when present ) with a velum;

   e. both fresh-water and marine .


 (2) Typical animal



    a. The structure of hydra ;

    b. The physiological function of hydra ;

    c. The life cycle of hydra .


Fig. Life cycle of Obelia, showing structure of the hydroid colony

5.4.2 Class Scyphozoa


(1) Characteristics of Class Scyphozoa


   a. solitary ;

   b. polyp stage reduced or absent ;

   c. bell-shaped medusae without velum ;

d. gelatinous mesoglea much enlarged ;

e. margin of bell or umbrella typically with eight notches that are provided with sense organs ;

f. all marine


2Typical animal

    Aurelia aurita    

a. The structure of Aurelia aurita

b. The life cycle of Aurelia aurita


Fig. The structure of Aurelia aurita


           male            sperm

              female       ovum      zygote


      ephyra                             blastula


         Asexual fission                   gastrula


       strobila                         planula


             Scyphistoma            settles


5.4.3 Class Anthozoa


  (1) characteristics


     a. all polyps, no medusae ;

     b. solitary or colonial ;

      c. enteron subdivided by at least 8 mesenteries or septa bearing nematocysts;

      d. gonads endodermal ;

      e. all marine .


(2) Typical animal



5.5 Metamorphosis


Generally, after swimming for a few hours to many days the planula attaches and develops into a polyp or polypoid form , which in colonial species subsequently gives rise to the colony .

In some hydroids the planula remains in the gonophore, developing into the  tentaculate actinula larva which is liberated and creeps about . After attachment, it develops into a polyp . In many hydrozoans with no polypoid phase the planula develops into an actinula and then a medusa .

In most scyphozoans, after attachment, the planula develops into a polypoid scyphistoma, with a stalked trumpet-shaped body . At maturity, the scyphistoma produces a free-swimming medusa stage, the ephyra larva, by transverse fission or strobilization. Ephyrae may be produced singly or several at a time, and develop to adult medusae . 

In zoantharian anthozoans, the planula does not attach but develops into an anemone-like Edwardsia larva, then the Halcampoides larva. After attachment, tentacles develop and the adult polyp form is attained.


5.6 Reproduction


There are both dioecious and hermaphroditic species .


5.6.1 Sexual reproduction


5.6.2 Asexual reproduction


Asexual reproduction is common, it may occur by :

a. budding ;

b. pedal laceration, e.g. in sea anemones ;

c. transverse fission, e.g. in the production of ephyrae by scyphistomae

d. longitudinal fission , e.g. in many sea anemones .


5.7 Polymorphism


In many cnidarians the life-cycle contains two morphologically dissimilar individuals , the pulp and the medusa . In colonial species, each of these types may occur in a number of different morphological forms , specialized to perform a particular function .

The main types of modified polyp are :

   a. the gastrozooid ---- feeding polyp

   b. the gonozooid ---- reproductive polyp

   c. the dactylozooid  ----       protective polyp , or tentaculozooid

a, b and c are found in hydrozoan colonies

   d. the autozooid ---- feeding and reproductive polyp

   e. the siphonozooid ---- current producing polyp

d, and e are found in some authozoan colonies

A colony may also bear medusoid forms in different stages of formation or degeneration, which may or may not be freed . Medusae may become modified as swimming bells, floats , protective bracts or phyllozooids, or gonophores which serve only for reproduction .



Phylum Ctenophora

About 100 species known . Ovoid forms measure up to about 5 cm , flattened forms may be up to 1 meter or more in length .


(1) Characteristics


a. symmetry biradial ; arrangement of internal canals and the position of the paired tentacles change the radial symmetry into a combination of the two ( radial + bilateral )

b. usually ellipsoidal or spherical in shape , with radially arranged rows of comb plates for swimming

c. Ectoderm , endoderm , and a mesoglea ( ectomesoderm ) with scattered cells and muscle fibers ; may be considered tripoblastic

d. Nematocysts absent ( except in one species ) , but adhesive cells ( colloblasts ) present

e. Digestive system consisting of mouth, pharynx , stomach , and a serious of canals

f. Nervous system consisting of a subepidermal plexus concentrated around the

mouth and beneath the comb plate rows; an aboral sense organ ( statocyst )

g. No polymorphism or attached stages

h. Reproduction monoecious, gonads ( endodermal origin ) on the walls of the digestive canals , which are under the rows of comb plates ; determinate cleavage ; cydippid larva

i. Luminescence common


(2) Comparison with Cnidaria


A. Ctenophores resemble the cnidarians in the following ways:


a. Form of radial symmetry ; together with the cnidarians , they form the group Radiata

b. Aboral - oral axis around which the parts are arranged

c. Well - developed gelatinous ectomesoderm

d. No coelomic cavity

e. Diffuse nerve plexus

f. Lack of organ systems


B. They differ from the cnidarians in the following ways:


a. No nematocysts except in Euchlora

b. Development of muscle cells from mesenchyme

c. Presence of comb plates and colloblasts

d. Mosaic, or determinate type of development

e. Presence of pharynx generally

f. No polymorphism


(3) Classification


A.      Class Tentaculata


With tentacles. Tentacles may or may not have sheaths into which they retract . Some types flattened for creeping ; other compressed to a bandlike form . In some the comb plates may be confined to the larval form.

B.      Class Nuda


Without tentacles; conical form; wide mouth and pharynx; gastrovascular canals much branched .



                            Review Questions


1.What characteristics of phylum Cnidaria are most important in distinguishing it from other phyla ?

2. Name and distinguish the classes in phylum Cnidaria.

3. Distinguish between polyp and medusa forms.

4. What is an un unusual feature of the nervous system of cnidarians?

5. Diagram a hydra and label the main body parts.

6. Name and give functions of the main cell types in the epidermis and in the gastrodermis of hydra.

7. Define the following with regard to hydroids: hydrocaulus, hydrorhiza, perisarc, hydranth, gonangium, manubrium, statocyst, ocellus, polymorphism, alternation of asexual and sexual stages.

8. Distinguish the following from each other: statocyst and rhopalium; scyphomedusae and hydromedusae; scyphistoma, strobila, and ephyrae; velum, velarium, and pedalium.

9. Define the following with regard to sea anemones: siphonoglyph; primary septa or mesenteries; incomplete septa; septal filaments; acontia threads; pedal laceration.

10. What characteristics of Ctenophora are most important in distinguishing it from other phyla?

11. Compare cnidarians and ctenophores, giving five ways in which they resemble each other and five ways in which they differ.

12. What is a widely held hypothesis on the origin of radiate phyla?