Cnidaria (Gr. Knide = nettle or stringing cell)

Classification of Animals Non Chordates of Class 11

Leuckart (1847) - Coined the name ‘coelentrata’.

Hatschek (1888) - Called them cnidaria

General characters

Name was given by Leuckart. Study of cnidarians is called cnidology.

Aristotle considered these animals as having plant like characters and called them Acalaphae.

These are diploblastic, acoelomate animals having cnidoblasts, blind sac body plan and radial symmetry.

Important characters

Coelenterata or Cnidaria is the phylum of diploblastic, acoelomate animals having cnidoblasts, blind sac body plan and radial symmetry. Number of species is about 9000.

Most of the coelenterates are marine but a few of them are fresh water.

Body form is various —vase-like, umbrella-like, branched or unbranched filament.

Coelenterates are diploblastic, i.e., develop from two germ layers.

The coelenterates possess cell-tissue level of organization.

Most of the coelenterates are colonial (e.g., Physalia), but some are solitary also (e.g., Hydra).

Coelenterates usually are found in more than one form and exhibit the phenomenon of polymorphism (i.e., in an individual more than one type of zooids are present). The two common forms are polyps and medusae. Polyp is usually fixed while medusa is free- living. Polyp is sessile. It is like a cylindrical stalk with mouth and tentacles facing upwards. Medusa is like an umbrella or bell with mouth and tentacles facing downwards. It is regarded as upside down polyp which can swim. In many forms polyps produce medusae by budding and medusae produce polyps after sexual reproduction. The animals have blind sac body plan. There is a single opening which serves both as mouth  and anus. It is commonly called oral aperture. The single aperture leads into a cavity called gastrovascular cavity or coelenteron. The latter may have septa. Symmetry is radial or biradial.

The oral end often bears thread like flexible tentacles.

Body wall is two layered, i.e., outer epidermis (ectoderm) and inner gastrodermis (endoderm). In between these two layers is present a non-cellular, nonliving gelatinous matrix or layer

called mesogl Sea. Branches of nerve cells may traverse mesogloea. A chitinous or calcareous exoskeleton may occur.

Epidermis    or surface    layer possesses    a number of cells — epithelio-muscular, glandulo-muscular (secrete adhesive materials/gas), germ cells, nerve cells, sensory cells, cnidoblasts and interstitial cells. Interstitial cells are reserve cells. Cnidoblasts are stinging cells. Epithelio-muscular cells form longitudinal muscles (for helping in extending body and tentacles).

Gastrodermis lines the gastrovascular cavity or coelenteron. It has five types of cells — nutritive (endothelio-muscular/gastrodermal)     cells, nerve cells, interstitial cells, secretory endothelio-glandular cells (mucus, digestive enzymes, lodge symbiotic algae), sensory cells. Nutritive cells may develop pseudopodia or flagella. They also form circular muscles (help in narrowing body and tentacles).

Nerve cells are apolar (nonpolar). They give out fibres which come in contact, and may get intertwined without developing protoplasmic continuity. They form networks or plexi. Some fibres are connected to pitheliomuscular cells and function as motor fibres. However, polarity in impulse transmission is absent. Sense organs (e.g., statocyst, tentaculocyst) occur in free swimming forms.

Digestion is both intracellular and intercellular or extracellular. Extracellular digestion occurs with the help of digestive enzymes poured into gastrovascular cavity by secretory cells. Partly digested food is engulfed by nutritive cells for intracellular digestion. The digested food is passed on to other cells by simple diffusion. Egestion occurs through mouth. Respiratory and excretory organs are absent as all the cells are in direct contact with water. Animals are ammonotelic.

In the life cycle of coelenterates usually asexual and sexual generations alternate regularly with each other. Both are diploid. This phenomenon is called alternation of generations or metagenesis. Asexual stage is usually polyp while sexual stage is medusa. Polyp reproduces by budding. It also forms sexual stage or medusa. Medusa is diploid. It produces haploid gametes through meiosis. Fertilisation produces diploid zygote. Zygote forms a free swimming mouthless ciliated larva called planula. The life cycle also possesses a number of immature stages like hydrula, hydratuba, scyphistoma (= scyphula), ephyra, etc.

Alternation of generations found in coelenterates is not similar to the one found in plants because in plants the asexual and sexual phases are diploid and haploid respectively.

Cnidoblasts :

Cnidoblasts, nematoblasts or stinging cells are organs of offence and defence of coelenterates.  Each cnidoblast bears a sensory bristle or cnidocil and a bladder or sac called nematocyst. The latter is connected to a contractile bundle of myonemes known as lasso. Each nematocyst has a long coiled tube with smooth, spiny or barbellate surface. There are 17 types of nematocysts, out of which four are common. (i)

Penetrant or Stenotele. It is the largest cnidoblast. Its nematocyst contains a poisonous proteinaceous fluid called hypnotoxin. The tube is long. It bears basal spines and stylets. The tube injects the fluid into victim for paralysing or killing (ii) Volvent or Desmoneme. It has a closed tube which can coil around projections of the victim to stop its movements (iii) Streptoline Glutinant ( = Large Glutinant = Holotrichous Isorhizas). The tubes bears spines but pours a sticky substance to adhere to the body of prey or support, (iv) Steroline Glutinant (= Small Glutinant = Atrichous Isorhizas). The thread is unarmed but otherwise takes part in glutination. It is the smallest cnidoblast.

Hydra (Fresh Water Polyp; Leeuwenhoek, 1703; Linnaeus, 1758)

It is a fresh water colourless carnivorous coelenterate having radial symmetry. Common species are H. gangeticus and H. vulgaris. Some hydras have green or brown colour due to the presence of zoochlorellae or zooxanthellae in their nutritive cells. Size is 5-30 mm. The body is tubular and contractile with an oral end and an aboral (foot) end. There is a large but simple gastrovascular cavity. Wall is two-layered, outer epidermis and inner gastrodermis. A noncellular gelatinous mesogloea occurs between the two. The body is covered by a thin cuticle. Epidermis is made of epithelio muscular cells, glandulo-muscular cells (adhesive material in pedal disc), interstitial cells, cnidoblasts, sensory cells (with delicate hair-like processes on free side), nerve cells and germ cells. Gastrodermis is formed of endothelio-muscular or nutritive cells, endothelio-glandular cells (two types, secreting digestive enzymes and secreting mucus), interstitial cells, sensory cells and nerve cells. The animal generally remains attached to submerged objects by means of foot or basal/pedal disc. The latter lacks mesogloea in central part. Hydra can, however, move about from one place to another by a variety of methods like somersaulting, looping, gliding, walking,  contains a ring of nerve cells and their branches. Growth zone occurs just below hypostome (Brien, 1963). Tentacles are rich in all types of cnidoblasts/nematoblasts. Hypostome has only streptoline or large glutinants. Cnidoblasts function as independent effector organs (Waren, 1960). Cnidoblasts seem to be activated in response to chemicals secreted by prey. They explode after coming in contact with prey. Nerve cells do not seem to exert influence over them. The body undergoes contraction and expansion that creates current in water and brings food nearer to it. Food is captured by means of tentacles. Lower

organisms lacking glutathione are not preyed upon. There is little digestion of starch. Hydra generally preys upon small crustaceans. Older cells are continuously replaced by new ones. All the cells get replaced in 45 days (Brien, 1955). Hydra has a great power of regeneration (Trembley, 1744). It develops buds (involving both body layers, exogenous) which grow into new individuals The animal also develops ectodermal gonads from interstitial cells (single ovary in proximal or lower half and a few testes in distal or upper half) for sexual reproduction. A few species are unisexual. Hermaphrodite forms are protandrous. Cross fertilization occurs. Development is direct. Gastrula is solid (= stereogastrula).

Phylum Coelenterata has been divided into 3 classes on the basis of dominance of polyp or medusa stage.

Class - 1 : Hydrozoa

1. Mostly marine, some fresh-water.

2. Polyp and medusa forms alternate in life cycle except Hydra.

3. Mesoglea- thin.

4. Gastrovascular cavity not divided into compartments by mesenteries.

Ex.- Hydra, Obelia, Physalia, Vellela & Porpita

Class - 2 : Scyphozoa

1. All marine.

2. Medusoid form predominant, polypoid form is reduced or absent.

3. Mesoglea - thick.

4. Ex. - Aurelia (Jelly fish or moonjelly) Rhizostoma, Cyanea (Sunjelly)

Class - 3 : Anthozoa  (Actinozoa)

1. All marine.

2. Only polypoid form is present, medusoid form absent.

3. Gastrovascular cavity divided into compartments by mesentries.

4. Includes sea anemones (Sea flower) and corals.

Ex. Metridium, Alcyonium , Gorgonia, Tubipora

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