Notes on PHYLUM CNIDARIA Or COELENTERATA

This phylum is also known as Coelenterata due to the presence of cavity known as coelenteron. This phylum takes its name due to the presence of cnidocytes, which contain organelles (cnidae) characteristics of the phylum. This phylum has the known species of almost 9000 and in some books, 11000 species are also mentioned.
 It includes plant like hydroid, jellyfish, sea anemone and coral etc. They may be in the form of a sessile polyp or freely floating medusa.

Cnidaria

Evolution of the nervous system was first time observed in cnidarians

Characteristics of  phylum Cnidaria

I.    Stinging cells called nematocysts housed by cnidocytes are found in the phylum.

II.    Most are marine but some are also present in fresh water.

III.    Possess radial or biradial symmetry (modification of radial symmetry).

IV.    Diploblastic and show tissue level organization.

V.    Mesoglea which is non-cellular, present between two body layers.

VI.    Incomplete gut called gastrovascular cavity.

VII.    Digestion is both intracellular and extracellular.

VIII.    Sense organs are statocysts (organs of balance) and ocelli (photosensitive organs).

IX.    No coelomic cavity.

X.    The nervous system is in the form nerve net.

Body wall in phylum Cnidaria

The cnidarian body comprises an outer epidermis, derived from the ectoderm and an inner gastrodermis derived from endoderm and mesoglea is present in between these two layers. The gastrodermis lines the gut cavity and functions mainly in digestion. In polyps of some hydrozoans, the epidermal layer has some types of cells including epitheliomuscular, interstitial, gland, sensory, nerve cells and cnidocytes are also present. The epitheliomuscular cells not only help in muscle contraction but also provide coverage.

The mesoglea is present between the epidermis and gastrodermis and is attached to both layers. It is a non-cellular and gelatinous material although it has some cells but these cells either have an epidermal or gastrodermal origin. In polyp, mesoglea form a continuous layer extending over both body and tentacles. It is thinnest in tentacles and thickest in stalk portion. This arrangement provides an ability to pedal region of the animal to withstand the mechanical strain and gives the tentacles more flexibility.

The mesoglea act as an elastic skeleton and help to support the body.

Cnidocytes

Many cnidarians are very effective predators and prey on larger and more intelligent animals than themselves and it is made possible because tentacles are armed with cnidocytes. Cnidocytes are borne in invagination of ectodermal cells and in some form are also developed in endodermal cells. Each cnidocyte produces one of over 20 kinds of distinctive organelles called cnidae that are discharged from the cell. During its development, a cnidocyte is properly called a cnidoblast. Once its cnida has been discharged, a cnidocyte is absorbed and replaced.

One type of cnida, the nematocyst is used to inject a toxin for prey capture and defense. Nematocysts are tiny capsules composed of material similar to chitin, containing a coiled hollow tube and a lid-like operculum that caps the capsule on top. Except in Anthozoa the cnidocytes has a modified cilium, called cnidocil. Stimulation of the cnidocil forces open the operculum and discharging the coiled tube.

The stimulus for the discharge of cnidoblasts is partly mechanical and partly chemical. Glutathione is a chemical released by small fishes causes the cnidocytes to be released, and mostly the toxin present in nematocyst is hypnotoxin.

3 important types of cnidae are:

Penetrant (these penetrate into the prey body and discharge toxin)

Volvents (these wrap around the prey)

Glutinants (these are sticky and binds with the prey)

Mechanism while discharging the nematocysts

Evidence indicates that nematocysts are discharged due to high osmotic pressure in the capsule, which may be 140 atmospheres. When nematocysts are stimulated to discharged, internal osmotic pressure causes the water to move into the capsule. The operculum opens and the increased hydrostatic pressure within the capsule forces the thread out with great force.

Forms and Alternation of generations

Most members of this phylum possess two body form in their life histories. One form is polyp while other is Medusa.

Polyp

A polyp is usually asexual and is adapted to sedentary or sessile life. Mostly polyps have tubular bodies and a mouth which is surrounded by tentacles, and it leads to a blind gastrovascular cavity. The aboral end of the body is attached to the substratum by pedal disc.

A polyp may reproduce asexually by budding, fission or pedal laceration.

Medusa

Medusa is free swimming and dioecious. It has an umbrella, bell or inverted bowl-like shape and tentacles dangle from its margins. The mouth opening is centrally located on the concave side of the body. Medusa swims by the gentle beating of the body wall. Medusa form has more mesoglea than the polyp form.

When a cnidarian life cycle involves both polyp and medusa stages then term alternation of generation (metagenesis) is used because one generation gives rise to other generation. Polyp by asexual reproduction gives rise to Medusa and Medusa has gonads where meiosis occurs and gametes are formed. These gametes fuse to form a polyp.

reproduction

Most cnidarians are dioecious. Sperms and eggs may be released into the gastrovascular cavity or to the outside of the body. In some cases, eggs are retained in the parent until after fertilization.

A blastula forms early in development, which later gives rise to the embryo. The embryo elongated to form a ciliated, motile planula larva. The larva attaches to substrate, its interior cells split to form a gastrovascular cavity and thus a young polyp is formed. Polyps always produce medusa by budding or by some other specialized methods like strobilation. In some species, medusa is not released from a polyp. it remains attached to it and forms gametes.