Phylum Coelenterata (Cnidaria) Definition:
Phylum Coelenterata (Cnidaria) are defined as diploblastic metazoa having a nematocyst and a single gastrovascular cavity, or coelenteron, as the tissue construction grade.
Phylum Coelenterata (Cnidaria) Characteristics
They are aquatic, except for a few freshwater species, such as the hydra.
- They are multicellular and organized at the tissue level.
- They are either single or conical. Inactive or free swimming.
- Individuals are radially or biradially symmetrical about a longitudinal oral-aboral axis.
- The majority of cells are dispersed and specialized for distinct purposes. Some cells form tissues such as nerve nets or nervous tissues.
- The exoskeleton is chitinous (perisarc) or calcareous(corals).
- They are diploblastic organisms with two cellular layers, an epidermis, and a gastrodermis, separated by an acellular gelatinous mesoglea.
- Acoelomate organisms lack a second bodily cavity known as the coelom.
- One or two whorls of short and delicate tentacles encircle the mouth.
- The tentacles are equipped with nematocysts; they are used for capturing food, ingesting it, adhering to surfaces, and defense.
- Attached sessile and asexual zooid (polyps) and free-swimming and sexual zooid are the two sorts of people (medusae). Some species are distinguished for their polymorphism or diversity of morphologies.
- They are typically carnivorous, with both extracellular and intracellular digestion.
- There is no anus.
- Coelom and deficiencies in the respiratory, circulatory, and excretory systems.
- The primitive nervous system is composed of a diffuse nerve network. Central nervous system missing.
- Epithelia-muscle cells and endothelial-muscle cells create longitudinal and circular muscle fibers in the muscular system.
- The mouth opens into a single gastrodermis-lined chamber termed the gastrovascular cavity or coelenteron.
- Sensory organs are composed of ocelli and statocysts.
- Reproduction occurs both asexually and sexually.
- The process of asexual reproduction is budding, while sexual reproduction is the generation of gametes.
- The development comprises a free-swimming planula larva with cilia.
- Life history is characterized by the phenomena of alternation of generation or metagenesis, in which the asexual, sessile polypoid generation alternates with the sexual, free-swimming medusoid generation.
Phylum Coelenterata (Cnidaria) Classification
- There are almost 11,000 recognized species in the phylum Coelenterata, half of which are extinct. This categorization was established by Hyman, L.H. (1940).
- According to the Hyman phylum, Coelenterata is divided into three groups.
Class 1. Hydrozoa (Gr., hydra=water + zoios=animal)
- Freshwater or saltwater? Unique or conical. Sessile or swimming freely.
- They display radial tetramerous and polymerous symmetry.
- The body wall is composed of the outer ectoderm, inner endoderm, and noncellular mesoglea.
- The gastrointestinal cavity is devoid of stomodaeum, septa, or nematocysts containing gastric filament.
- In some forms, the skeleton or horny structure is horny perisarc, but in other forms, coenosarc produces a calcium carbonate skeleton, forming a massive stony structure or coral.
- There is polymorphism present. The asexual polyp and the sexual medusa are the two basic forms of zooids.
- Lack of stomodaeum and septa in a polyp (mesentery).
- Medusa with authentic velum (Craspedote).
- Mesoglea non-cellular.
- Many of them display generational alternation.
- Gonads are epidermal. Sex cells are expelled externally.
- Holoblastic cleavage, ciliated embryo in planula.
Order 1: Hydroida
- Unique or conical.
- The polypoid phase predominates.
- Medusae are transient or nonexistent.
- The sense organs of medusae are ocelli and statocysts of ectodermal origin only.
Suborder 1. Anthomedusae or Athecata
- unique, or conical.
- Polyps and blastostyles athecate, meaning that the perisarc does not produce hydrothecae and gonothecae.
- Medusae are tall, bell-shaped creatures with gonads on the manubrium and an umbrella with a strong arch.
- Medusae have eyespots, or ocelli, at the base of their tentacles.
- Statocysts are not present.
- Examples: Hydra, Ceratella, Tubularia, Clava, Eudendrium.
Suborder 2. Leptomedusae or Thecata
- Conical Hydrozoa
- Polyps are surrounded by hydrothecae, whereas medusae are covered by gonothecae.
- Flattened, bowl-or saucer-shaped medusae with gonads on the radial canal.
- Medusa’s gonads are located in radial canals.
- Typically, medusae have statocysts.
- Eyespots, or ocelli, are missing.
- Examples: Obelia, Sertularia, Plumularia, Aglaophenia.
Order 2: Milleporina
- Conical Hydrozoa lacks a perisarc, resembling coral.
- The ectoderm produces an enormous calcareous skeleton with holes from which polyps emerge.
- There are two types of zooids in a colony: the gastrozooid and the dactylozooid.
- Gastrozooids (nutritional zooids) have small mouths and tentacles.
- Dactylozooids are hollow, elongated, and thin, with tentacles but no mouth.
- In tiny chambers, medusae grow into free creatures devoid of mouths, radial canals, and tentacles.
- Example: Millepora.
Order 3: Stylasterina
- There are two types of zooids seen in coral-like Hydrozoa colonies: dactylozooids and gastrozooids.
- Dactylozooids are tiny, solid, and limbless.
- Gastrozooids have a cup-shaped body with a sharp spine.
- Reduced gonophores to sporosacs. Medusae are captive.
- Planula are released as larvae.
- Stylaster, for example.
Order 4. Trachylina
- Polypoid stage is diminished or missing.
- Medusae are enormous, dominating, free-swimming creatures that can evolve from fertilized eggs.
- Statocysts or marginal sense organs with endodermal statoliths,
Suborder 1: Trachymedusae
- Tentacles positioned over the bell edge
- The umbrella’s border is smooth.
- Manubrium is lengthy.
- Gonads arise in radial canals.
- Example: Geryonia.
Suborder 2. Narcomedusae
- Tentacles emerge between the exumbrella’s bell border and apex.
- The manubrium is short in length.
- The presence of gonads on the manubrium or stomach floor.
- Examples: Cunina, Solmaris.
Order 5. Siphonophora
- They are colonies of polymorphic, free-swimming, or floating Hydrozoa.
- Several forms of polypod and medusoid individuals are linked to the colony’s stem or disc.
- Oral polyps are devoid of oral tentacles.
- Medusae are fragmentary and hardly released.
Suborder 1. Calycophora
- The uppermost portion of the colony is equipped with at least one swimming bell (nectophores).
- Apical float or absence of Pneumatophore.
- Examples: Diphyes, Praya, Abyla.
Suborder 2. Physophorida
- The colony’s upper section produces a vast, gas-filled float (pneumatophore).
- Examples: Physalia, Halistemma, Stephalia.
Class 2. Scyphozoa (Gr., skyphos=cup +zoios=animal)
- It contains big, marine-exclusive medusae, or real jellyfish.
- Large, bell- or umbrella-shaped, without real velum, free-swimming, or linked by an aboral stalk, medusae lack genuine velum and have no true velum.
- Reduced polyp stage or absence.
- Tentaculocysts with endodermal statoliths compose marginal sense organs.
- Gastrovascular canal with endodermal gastric filaments and gastric pouches.
- No stomodaeum.
- Extensive, gelatinous, and composed of fibers and cells, Mesoglea.
- Gonads are composed of gastrodermal tissue. Sexually active cells are discharged into the digestive tract.
Order 1. Stauromedusae or Lucernaridae
- Goblet or trumpet-shaped body.
- Sessile, with an aboral stalk for attachment.
- The mouth is cruciform with modest oral lobes and a short quadrilateral manubrium.
- Four inter-radial septa separate the gastrovascular system into the core stomach and four per-radial pouches.
- On the faces of septa, gonads appear as elongated bands.
- There are no peripheral sensory organs or tentacles.
- Fertilization from outside.
- The larva is a cilia-less planula.
- Examples: Lucernaria, Haliclystus.
Order 2. Cubomedusae or Carybdeida
- Cubic body with four flattened sides.
- Free-swimming Scyphozoa that inhabit warm, shallow tropical and subtropical seas.
- Four interradial tentacles that are hollow are carried along the edge of the sub-umbrella.
- 4 per-radial tentaculocysts, or rhopalia, are found.
- Each tentaculocyst contains a lithocyst and at least one ocelli.
- There are cruciform gastric pouches present in the mouth.
- Leaf-like gonads.
- For example, Charybdis and Tamoya.
Order 3. Coronate
- Conical body separated by a deep, circular coronary groove
- Scyphomedusae inhabits the ocean’s deep waters and swim freely.
- A coronal groove (horizontal furrow) divides the umbrella into an upper cone and a lower crown.
- The crown is composed of pedalia, or pedal lobes.
- The pedalia have robust tentacles.
- The border of the bell is scalloped into alternate lappets and pedalia.
- Cruciform oral shape
- Four to sixteen tentaculocysts are found.
- Examples: Pericolpa, Periphylla.
Order 4: Semaeostomeae
- The most prevalent free-swimming medusae are found in the coastal waters of all seas.
- The form of the umbrella is flat, saucer, or bowl.
- A square-shaped mouth with four long oral limbs.
- The umbrella’s edging is fringed with hollow tentacles.
- Eight tentaculocysts are present.
- The absence of gastric pouches and filaments
- Examples: Aurelia, Cynaea.
Order 5. Rhizostomae
- Typically, the body is a hemisphere without peripheral tentacles.
- Free-swimming Scyphozoa are found in tropical and subtropical shallow seas.
- The top of the umbrella is saucer- or bowl-shaped, flattened, or even concave.
- The mouth is encompassed by eight oral arms with multiple funnel-shaped mouths along their edges.
- Typically at least eight tentaculocysts.
- Four subgenital pits are frequently present.
- Examples: Rhizostoma or Pilema, Cassiopeia.
Class 3. Anthozoa (Gr., anthos= flower+ zoios= animal)
- Exclusively aquatic Unique, or conical.
- Absolutely polypoid
- No medusoid stage.
- Typically, the body has hexamerous, octamerous, or polymerous biradial or radobilateral symmetry.
- The oral end of the animal is radially extended into an oral disc with tentacles encircling the mouth’s center.
- The stomodaeum is often equipped with one or more siphonoglyphs, which are ciliated grooves.
- The gastrovascular cavity is subdivided by at least eight septa or mesenterie
- The inner free borders of the mesentery contain nematocysts.
- Mesoglea has fibrous connective tissue and amoeboid cells.
- Either an exterior or an interior skeleton.
- The exoskeleton is composed of calcium carbonate, which frequently creates coral reefs.
- The nervous system resembles a conventional nerve network but lacks a central nervous system.
- The endodermal gonads form in the mesentery.
- The mature sexual products are released into the coelenteron.
- External fertilisation
- The fertilized egg becomes a planula larva, which, after a brief period of freedom, settles down and matures into an adult.
Subclass 1. Alcyonaria or Octocorallia
- Unquestionably colonial
- Polyps are long or short cylinders that end orally in a flat, circular oral disc with an oval or elongated mouth in the middle.
- A polyp has eight tentacles and eight septa.
- Eight complete mesenteries are present.
- Single ventral siphonoglyph is present.
- Endoskeleton is composed of calcareous or horny spicules and is produced by mesogleal cells.
- Polyps are in some way dimorphic.
Order 1. Stolonifera
- Tropical and temperate area inhabitants of shallow water.
- Polyps that develop independently from a mat or stolon.
- Calcareous tubes, spicules, or absence of a calcareous skeleton
- Examples: Tubipora, Clavularia.
Order 2. Telestacea
- The colony consists of unbranched or branched branches that emerge from a creeping base.
- Each stem is a polyp with lateral polyps that are extremely elongated.
- The skeleton is made up of calcareous spicules.
- Example: Telesto.
Order 3, Alcyonacea
- Colonies formed like mushrooms or branched into robust, blunt processes.
- One oral end protrudes from the fleshy mass, or coenenchyma, formed by the fusion of the polyp’s lower halves.
- In some forms, polyps are dimorphic and contain both autozooids and siphonozooids.
- The skeleton is composed of distinct clarous spicules, not axial bones.
- Examples: soft corals. Alcyonium, Xenia.
Order 4. Coenothecalia
- Not fused spicules, crystalline calcareous fibers of calcium carbonate make up the majority of the skeleton.
- Embedded polyps are linked by solenial tubes.
- Blue corals are corals found in Indo-Pacific coral reefs and are commonly referred to by this name.
- Example: Heliopora (blue coral).
Order 5. Gorgonacae
- The colony is often plant-like, with the main stem originating from the basal plate or tuft of stolon and a number of polyp-bearing branches.
- An axial skeleton is comprised of gorgonin, separate or fused spicules, or both.
- Typically referred to as sea fans, sea feathers, or sea whips.
- on the tropical and subtropical coastlines.
- Examples: Gorgonia, Corallium.
Order 6. Pennatulacea
- Colony stretched and separated into a proximal stalk or peduncle and a distal rachis.
- Their peduncles were immersed in dirt and sand.
- The upper portion (rachis) is comprised of a very long axial polyp with lateral branches bearing a polyp that is dimorphic.
- The main stem is supported by a calcareous or horny skeleton.
- Examples: Pennatula, Renilla, Cavernularia, Pteroides.
Subclass 2. Zoantharia or Hexacorallia
- Individual or colonial?
- The marine form
- Tentacles are typically simple and unbranched, organized into multiples of five and six, but seldom eight.
- Numerous mesenteries are organized into multiples of 5 or 6, and maybe complete or insufficient.
- Typically, the gullet has two siphonoglyphs.
- When present, the endoskeleton is calcareous and derived from the ectoderm.
- Typically, a monomorphic polyp.
Order 1. Actiniaria
- Individual or colonial?
- Typically, simple things are enormous in size.
- No skeleton.
- Muscular body, frequently with an aboral pedal disc.
- Numerous tentacles and mesentery are present.
- Multiple siphonoglyphs
- Examples: Actinia, Metridium, Adamsia, Edwardsia.
Order 2. Madreporaria
- Rarely solitary and predominantly colonial
- The exoskeleton is typically rigid, compact, and massively calcified.
- Small polyps inhabit cup-like spaces on the exoskeleton.
- No siphonoglyphs and weak muscles.
- Examples: genuine or stony coral. Astraea (star coral), Fungia, Favia, Madrepora (staghorn coral), and Meandrina (brain coral).
Order 3: Zoanthidea
- Typically colony, occasionally solitary types.
- There is neither a skeleton nor a pedal disc, but the body wall includes calcareous bodies.
- mostly epizoic
- Typically, small polyps are joined by basal stolons.
- matched mesentery A pair consisting of a full mesentery and an incomplete mesentery
- Exists only one ventral siphonoglyph.
- Example: Zoanthus.
Order 4: Antipatharia
- Colonial and arboreal.
- Located in the ocean’s deep waters.
- Typically, the base of the colony consists of a basal plate for the attachment of various things.
- Tentacles and mesentery numbers are rather low (6-24).
- The skeleton is a branching, ectoderm-derived chitinoid axis.
- The axial backbone carries dioecious polyps, yet the colony may be hermaphrodite.
- Two siphonoglyphs exist.
- Examples: Black corals. Antipathes.
Order 5. Ceriantharia
- Long, solitary, anemone-like organisms inhabit the vertical cylinder chambers of the ocean floor.
- No pedal disc or skeleton.
- With a smooth, cylindrical, elongated body and an oral disc.
- Simple, many tentacles organized into two whorls-oral and marginal.
- Individual and dorsal siphonoglyphs
- Multiple, solitary, and full mesenteries exist.
- Instance: Cerianthus.
- Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
- Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.