• Cnidarian have reached a define tissue level of organization
  • They are diploblastic i.e. the body wall consist of two basic layers: an outer ectoderm and an inner called the endoderm lining the gastro vascular cavity (enteron) however in between the two layers is an acellular (without cell) mesoglea, with no known function
  • They are acoelomate with radial symmetry
  • They are majorly free living organisms comprising of sedentary/sessile (i.e attached to fixed point motionless) and free swimming forms.
  • Locomotion in the free swimming forms by means of tentacle
  • Possession of diffuse network of neuron which forms the nervous system
  • Possession of a sting capsule called the nematocyst which is found in either or both layers for offence and defense
  • Presence of remarkable degree of polymorphism aphenenor whereby there are different individuals belonging to the same specie or organism
  • There are two different structural types or polymorphic form: the polyp usually sessile and the medusa or medusoid which is usually free swimming with reproductive functions.
  • Absence of respiratory, circulatory and excretory system
  • The exoskeleton in few members may be composed of calcium carbonate of horny material while large vast of the member may lack skeleton
  • Reproduction takes place asexually by budding e.g in the polyp form or sexually by cross fertilization.
  • Example include hydra, obelia, Aurelia

Characteristics of cnidarians

  • They are diploblastic animals:body well composed of two layers of cells, an outer ectoderm and an inner endoderm;these layers are separated by a structureless, gelatinous layer of mesoglea which may contain cells thathave migrated from other layers.
  • They are at tissue level oforganisation.
  • Radially symmetrical
  • Their body is basically sac-shapedwith a single opening, the mouth, for ingestion and egestion. The single cavitywithin the sac is called the enteron andis where digestion takes place.
  • Consists two structural types,polyps and medusa. Polyps are sessile (stay in one place) and may be solitary,e.g. Hydra, or colonial, e.g. obelia. Medusa are free-swimming andsolitary.
  • They exhibit Polymorphism, that is, individuals havespecialised shapes with different functions – a form of division of labour.


The body plan is relatively simple,consisting of two layers of cells, an ectoderm and an inner endoderm. This isknown as the diploblastic level of organisation. The ectodermal cells faceoutwards into the environment and the endodermal cells face inwards into theenteron, a cavity with a single opening to the environment, the ‘mouth’.

Feeding is by means of tentacles arrangedaround the mouth. Both ingestion (taking in food) and egestion (getting rid ofundigested food) take place through this opening. There is some specialisationof cells, so it can be argued that the tissues level of organisation has beenachieved.

For example, batteries of spitting cellsknown as nematoblasts occur in theectoderm of the tentacles. These can discharge threads of three types which canpenetrate, cling to or kill the prey.

The ectoderm also contains sensory cells,which connect with nerve cells, forming a communication network through themesoglea. Cells containing contractile, muscle-like fibrils allow movement ofthe body and tentacles, and locomotion in jellyfish. In the endoderm are cellsspecialised for various aspects of digestion and absorption.

Radial symmetry

Cnidarians are radially symmetrical,meaning that they can be cut in half across any diameter and the two halveswill be identical (like a cake). Radial symmetry tends to be associated withorganisms which do not show locomotion.

Most animals show bilateral symmetry(associated with locomotion) which not only gives a more compact andstreamlined shape but allows greater specialisation of body parts.

Polyps, medusa and polymorphism

Two basic body types occur in thecnidarians, the polyp is cylindricaland sessile. A sessile organism isone that remains attached to a surface such as rock throughout its life andshows little or no locomotion. The medusais umbrella-shaped and free-swimming or floating.

The two types sometimes alternate in thelife cycle, in which case the medusa can act as a dispersal stage. In thissituation polyps reproduce asexually by budding off medusa, and the medusareproduce sexually to produce larvae which develop into polyps.

Individual polyps within colonies can alsovary in form. For example, they may be specialised for feeding or for asexualreproduction. The situation in which individuals of a species exist in two ormore different forms is known as polymorphism.

Size of cnidarian

Cnidarians are still relatively smallanimals. The few large jellyfish consist mainly of mesoglea which is not madeof living cells. With only two layers of cells, nutrients can diffuse rapidlyfrom the feeding cells in the endoderm to ectoderm.

In addition, all cells are in directcontact with the water of the environment, so gaseous exchange can take placevery efficiently by diffusion. The organism has a large surface area to volumeratio.

Obelia, Aurelia and actinia are all marinespecies (live in sea). Obelia is a good example of polymorphism, with colonialpolyps alternating with a small jellyfish stage in the life cycle. It is commonin shallow waters attached to rocks, shells, the fronds of large seaweeds orpiers. Actinia is very common around the shores of Britain, particularly insheltered places such as cracks in rock pools.

Classification of phylum cnidaria

The three classes of cnidaria along withtheir characteristic features and examples are given in the table below.

Class Hydrozoan (hydroids) Class Scyphozoa (jellyfish) Class Anthozoa (corals and sea anemones)
Polyp dominant in life cycle Small polyp sometimes present as a larval stage Polyp only – more complex than those of the Hydrozoa
Medusa simple Large highly organised medusa dominant in life cycle No medusa
Polyps solitary or colonial   Polyps solitary (anemones, some corals) or colonial (most corals)
Nematoblasts (stinging cells) Nematoblasts Nematoblasts
e.g. Hydra (no medusa phase), Obelia e.g. Aurelia (jellyfish) e.g. Actinia (beadlet anemone) Madrepora (coral)