Phylum Coelenterata: Definition, Characteristics, Classification and Examples

Table of Contents

PHYLUM COELENTERATA

Phylum Coelenterata (Gr., koilos = hollow ; enteron = intestine), also known as Phylum Cnidaria, is a group of aquatic, mostly marine animals characterized by a simple body structure with a single opening serving as both mouth and anus. Members of this phylum have a diploblastic body (two tissue layers: ectoderm and endoderm) and radial symmetry. They possess specialized stinging cells called cnidocytes containing nematocysts, used for defense and capturing prey. Coelenterates exhibit two main body forms: the sessile polyp (e.g., Hydra) and the free-floating medusa (e.g., jellyfish). This phylum includes organisms like corals, sea anemones, and hydroids, playing important roles in marine ecosystems, such as reef-building.

General characteristics of Phylum Coelenterata

Coelenterates are metazoan or multicellular animals with tissue grade organization.
They are aquatic, mostly marine except few freshwater forms, e.g., Hydra.
Solitary or colonial. Sedentary or free-swimming.
Individuals are radially or biradially symmetrical with a central gastrovascular cavity communicating with the exterior by mouth. Anus is absent.
Short and slender tentacles encircle the mouth in one or more whorls. The tentacles serve for food capture, its ingestion and for defence.
Body wall diploblastic with two cellular layers – outer epidermis (or ectoderm) and inner gastrodermis (or endoderm) – with a gelatinous acellular mesogloea in between.
They are acoelomate animals because they do not possess a second body cavity, the coelom.
They exhibit the phenomena of polymorphism. Two main types of structures, the polyp and medusa are formed.
Polyp is sessile and asexual zooid, while the medusa is free-swimming and sexual zooid.
Exoskeleton chitinous (perisarc) or calcareous (corals).
Coelenterates possess nematocysts which serve the function of paralysing the prey by injecting poison or to hold the prey.
Reproduction both sexual and asexual.
Asexual 🡪 Budding
Sexual 🡪 Sperm and Ova
The egg usually develops into a ciliated larva known as planula.
Exhibits the phenomena of alteration of generation.

Classification of Phylum Coelenterata

Note: Classification is adopted from Hyman, L.H. (1940)

CLASS 1. HYDROZOA

Hydrozoa are solitary and freshwater or mostly colonial and marine, sessile and free-swimming forms.
They exhibit tetramerons or polymerous radial symmetry.
Body wall consists of an outer ectoderm and an inner endoderm separated by a non-cellular gelatinous mesogloea.
Gastrovascular cavity without stomodaeum, septa or nematocysts bearing gastric filament.
Skeleton or horny structure is horny perisarc in some forms, while coenonarc secretes a skeleton of calcium carbonate forming massive stony structure or coral in other forms.
They exhibit polymorphism. There are two main types of zooids, the polyp and medusa. Medusa is provided with true muscular velum.
Many Hydrozoa exhibit alternation of generation.
Reproductive products or sex cells are usually ectodermal in origin and discharged externally.
Cleavage is holoblastic, embryo ciliated planula.

Order 1. Hydroida

Solitary or colonial forms.
Polypoid generation well-developed gives rise to free or abortive medusae by budding.
Sense organs of medusae are ocelli and statocysts and exclusively ectodermal in origin.

Suborder 1. Anthomedusae or Athecata

Solitary or colonial.
Polyps are not enclosed in hydrothecae and the medusae are also naked without gonotheca.
Medusae are tall, bell-like, bearing gonads on the manubrium, having strongly arched umbrella.
Medusae bear eye spots or ocelli at the bases of tentacles. Statocysts are absent.
Examples: 1. Hydra, 2. Tubularia, 3. Bougainvillea, 4. Hydractinia, 5. Eudendrium, 6. Pennaria, 7. Ceratella.

Suborder 2. Leptomedusae or Thecata

Colonial Hydrozoa.
Polyps are enclosed in hydrotheca and medusae are covered with gonotheca.
Free medusae are flattened, bowl or saucer-shaped, bearing gonads on the radial canals.
Medusae usually bear statocysts. Eye spots or ocelli are absent.
Examples: 1. Obelia, 2. Sertularia, 3. Plumularia, 4. Campanularia

Order 2. Milleporina

Colonial coral-like Hydrozoa without perisarc.
Massive calcareous skeleton is secreted by ectoderm provided with pores through which polyps protrude out.
Colony has two kinds of zooids, the gastropozooids and the dactyloozooids.
Gastropozooids (nutritive zooids) are short provided with mouth and tentacles.
Dactyloozooids are elongate, hollow slender with tentacles but without mouth.
Medusae develop in small chambers, becoming free, devoid of mouth radial canals and tentacles.
Example: Millepora

Order 3. Stylasterina

Colonial coral-like Hydrozoa.
Colony has two kinds of zooids, the dactyloozooids and gastropozooids.
Dactyloozooids are small, solid without tentacles.
Gastropozooids have a cup with pointed spine.
Medusae develop in special cavities, reduced to sporosacs.
Larva is liberated as planula.
Example: Stylaster

Order 4. Trachylina

Polypoid stage reduced or absent.
Medusae are large provided with tentaculocysts, statocysts and lithocysts enclosed in the endoderm.

Suborder 1. Trachymedusae

Margin of the umbrella is smooth.
Manubrium is long.
Gonads borne on the radial canals.
Examples: 1. Geryonia, 2. Gonionemus

Suborder 2. Narcomedusae

Margin of the umbrella is scalloped or clefted by tentacle’s bases.
Manubrium borne in the floor of the stomach.
Gonads borne in the floor of the stomach.
Examples: 1. Cunina, 2. Solmaris

Order 5. Siphonophora

Siphonophora are polymorphic, free-swimming or floating colonial Hydrozoa.
Colony consists of several types of polypoid and medusoid individuals attached to stem or disc.
Polyps without tentacles.
Medusae always incomplete and rarely freed.

Suborder 1. Calycophora

The upper end of the colony is provided with one or more swimming bells.
Apical float or pneumatophore absent.
Example: Diphyes

Suborder 2. Physophorida

Upper end of the colony bears a float or pneumatophore.
Examples: 1. Physalia, 2. Velella, 3. Porpita, 4. Halistemma

CLASS 2. SCYPHOZOA

Scyphozoa include large jelly-fishes or true medusae, are exclusively marine.
Medusae are large, bell or umbrella-shaped, without true velum, free-swimming or attached by an aboral stalk.
Marginal sense organs are tentaculocysts having endodermal statoliths.
Polypoid generation absent or represented by small polyp, the scyphistoma which gives rise to medusae by strobilization or transverse fission.
Gastrovascular system without stomodaeum, with gastric filaments and may or may not be divided into four inner-radial pockets by septa.
Mesogloea is usually cellular.
Gonads are endodermal and the cells are discharged into the stomach.

Order 1. Stauromedusae or Lucernaridae

Sessile, sedentary Scyphozoa attached by an aboral stalk.
Body globe or trumpet-shaped.
Mouth cruciform (four-cornered) with small oral lobes and a short quadrangular manubrium.
Gastrovascular system is divided into four inter-radial septa.
Gonads are elongated, band-like borne on the faces of septa.
Marginal sense organs absent.
Fertilization is external. Larva is planula without cilia.
Examples: 1. Lucernaria, 2. Haliclystus

Order 2. Cubomedusae or Carybdeida

Free-swimming Scyphozoa found in warm and shallow waters of tropical and subtropical regions.
Body cubical with four flat sides.
Four hollow inter-radial tentacles borne on the margin of the sub-umbrella.
Four per-radial tentaculocysts or rhopalia are present. Each tentaculocyst is provided with a lithocyst and one or more ocelli.
Mouth is cruciform and gastric pouches are present.
Gonads are leaf-like.
Examples: 1. Charybdaea, 2. Tamoya, 3. Chiropsalmus

Order 3. Coronate

Free-swimming scyphomeduase found inhabiting the deep water of ocean.
Body conical, dome-shaped or flattened, grooved.
The umbrella is divided by coronal groove (horizontal furrow) into an upper cone and a lower crown.
The crown consists of pedal lobes, pedalia. The pedalia bear solid tentacles.
The bell margin is scalloped into lappets alternate with pedalia.
Mouth is cruciform.
Tentaculocysts are four to sixteen.
Examples: 1. Pericalpa, 2. Periphylla

Order 4. Semaeostomeae

Most common free-swimming medusae found inhabiting the coastal waters of all oceans.
The umbrella is flat, saucer or bowl-shaped.
Mouth is square. The corners of the mouth produce four oral arms which are grooved with frilled edges.
The margin of the umbrella is fringed with hollow tentacles and eight or more tentaculocysts are present.
Gastric pouches and filaments are absent.
Examples: 1. Aurelia, 2. Cyanea

Order 5. Rhizostomeae

Free-swimming Scyphozoa found in shallow waters of tropical and subtropical oceans.
The umbrella is saucer or bowl-shaped or flattened or even concave on the top.
Mouth is surrounded by eight oral arms bearing numerous funnel-shaped mouth on their edges.
Marginal tentacles are absent but 8 or more tentaculocysts are present.
Four subgenital pits are generally present.
Examples: 1. Rhizostoma or Pilema, 2. Cassiopeia

CLASS 3. ANTHOZOA

Solitary or colonial exclusively marine forms.
They are exclusively polypoid. Medusoid stage is altogether absent.
Body usually cylindrical with hexamerous, octomerous or polymerous biradial or radiobilateral symmetry.
The oral end of the body expanded radially into an oral disc bearing tentacles surrounding the mouth in the centre.
The stomodaeum is present often provided with one or more ciliated groove the siphonoglyphs.
Gastrovascular cavity is divided into compartments by complete or incomplete septa or mesenteries.
Mesenteries bear nematocysts at their inner free edges.
Mesogloea contains fibrous connective tissue and amoeboid cells, forming massive corals.
Skeleton either external or internal. Exoskeleton is formed from calcium carbonate which often form massive corals.
Nervous system is in the form of typical nerve net without a concentrated central nervous system.
Gonads are endodermal, develop in the mesenteries. The ripe sexual products are discharged into coelenteron.
Fertilization is external. The fertilized egg develops into a planula larva, which after a short free-life, settles down and develops into an adult.

Subclass I. ALCYONARIA OR OCTOCORALLIA

Colonial marine forms.
Polyps are long or short cylinder terminating orally into a flat circular oral disc having the oval or elongated mouth in the centre.
Polyps always bear eight pinnate hollow tentacles.
Eight complete mesenteries are present.
Single ventral siphonoglyph is present.
Endoskeleton is the product of mesogleal cells comprised calcareous spicules either calcareous or horny in nature.
Polyps are dimorphic in some forms.

Order 1. Stolonifera

The Stolonifera are inhabitants of shallow waters in the tropical and temperate regions.
Polyps arise singly from a creeping base and are connected by one or more solenial tubes.
Skeleton consists of loose spicules or of compact tubes and platforms.
Examples : 1. Tubipora, 2. Clavularia

Order 2. Telestiacea

Colony consists of simple or branched stems arising from a creeping base.
Each stem is very elongated polyp bearing lateral polyps.
Skeleton consists of spicules fused by calcareous or horny secretions.
Example : Telesto

Order 3. Alcyonacea

Colony mushroom-shaped or branched into stout blunt processes.
Lower part of the polyp fused into a fleshy mass with only oral ends protruding.
Polyps are dimorphic in some forms, bearing autozooids and siphonozooids.
Skeleton consists of separate calcareous spicules, not axial.
Examples: 1. Alcyonium, 2. Xenia, 3. Hartea

Order 4. Coenothecalia

Skeleton is massive, composed of crystalline calcareous fibres of calcium carbonate, not of fused spicules.
Skeleton is perforated by numerous larger and smaller erect cavities. The larger ones contain the lower ends of the polyps, while the smaller ones the erect terminal tubes.
Commonly known as blue corals found on the coral reefs in the Indo-Pacific.
Example: Heliopora

Order 5. Gorgonacea

The colony is usually plant-like consists of a main stem arising from a basal plate or tuft of stolons and a number of branches bearing polyps.
The axial skeleton composed of calcareous spicules or horn-like material, gorgonin is present.
Commonly known as sea fans, sea feathers and sea whips are found in tropical and subtropical shores.
Examples: 1. Gorgonia, 2. Corallium

Order 6. Pennatulacea

The colony is elongated and divided into a proximal stalk or peduncle and a distal rachis.
Peduncle is embedded in the mud or sand.
Rachis is the axial polyp bears numerous dimorphic polyps on its lateral branches.
The main stem is supported by a calcareous or horny skeleton.
Examples: 1. Pennatula, 2. Renilla, 3. Cavernularia, 4. Ptilosarcus

Subclass II. ZOANTHARIA OR HEXACORALLIA

Solitary or colonial marine forms.
Tentacles simple, rarely branched, hollow cone-shaped, numerous arranged in the multiple of five or six but never eight.
Mesenteries are numerous arranged in the multiple of five or six, may be complete or incomplete.
Two siphonoglyphs are commonly present.
Endoskeleton when present is calcareous, derived from ectoderm.
Polyps are usually monomorphic.

Order 1. Actiniaria

Solitary or colonial.
Body cylindrical, divided into oral disc, column and base. The aboral end is also provided with a pedal disc.
Tentacles and mesenteries are numerous and often arranged in the multiple of six.
Siphonoglyph is one or more.
Skeleton absent.
Examples: 1. Actinia, 2. Metridium, 3. Adamsia, 4. Edwardsia, 5. Minyas

Order 2. Madreporaria

Mostly colonial rarely solitary forms.
Exoskeleton is hard and calcareous secreted by the ectoderm.
Polyps are small enclosed in the cup-like cavities of the exoskeleton.
Siphonoglyphs are usually absent.
Examples: 1. Astraea, 2. Fungia

Order 3. Zoanthidea

Mostly colonial sometimes solitary forms.
Skeleton and pedal disc absent, but the body wall may contain calcareous bodies.
Polyps are generally small.
Mesenteries are paired. A pair composed of one complete and one incomplete mesentry.
Only one ventral siphonoglyph present.
Example: Zoanthus

Order 4. Antipatharia

Plant-like colonial forms found in the deep waters in the oceans.
The lower end of the colony usually consists of a basal plate for the attachment with some objects.
Skeleton in the form of a branched chitinoid axis derived from the ectoderm.
The axial skeleton bears the polyps which are dioceous but the colony may be hermaphrodite.
Mesenteries and tentacles are 6–24 in number.
Two siphonoglyphs present.
Example: Antipathes

Order 5. Ceriantharia

Long, solitary anemone-like forms living in the vertical cylindrical cavities in the sea bottom.
Body smooth cylindrical and elongated with an oral disc.
Pedal disc and skeleton absent.
Tentacles are simple numerous and of two types, marginal and oral.
Mesenteries are numerous, single and complete.
Only single dorsal siphonoglyph present.
Example: Cerianthus