Body wall of all the coelenterates contains special defensive structures called as stinging cells or nematocysts.
Cnidocyte contains a fluid-filled membranous capsule called cnida.
Help not only in defense but also in locomotion, adhesion and capture of prey.
Cnidoblasts develop only from modified interstitial cells of epidermis and are not found in the gastrodermis.
When fully developed, cnidoblasts migrate to the tentacles through mesoglea by means of amoeboid movement.
Structure of cnidoblast
Cnidoblast is an oval or rounded cell with a basal nucleus on one side.
Inside the cnidoblast an oval called Nematocyst (stinging capsule) is present.
Nematocyst consists of a tiny bulb made of chitin.
Bulb is filled with poisonous fluid or hypnotoxin, which is chemically a mixture of proteins and phenols.
One end of this bulb is extended as a narrow, long, hollow tube like filament which is coiled round the poisonous sac (Thread tube).
Base of the thread tube is swollen to form a shaft.
Inside the shaft there are three large spines called as barbs and three spiral rows of minute spines called as barbules.
Shaft is externally covered by a lid-like structure called as operculum.
Outer end of the cnidoblast projects freely beyond the epidermal surface as a tiny, pointed hair-like process called cnidocil or trigger.
Groups of supporting rods surround the central core of cnidocil.
Cytoplasm of the cnidoblast contains contractile muscle fibrils.
Distribution of nematocyst
Nematocysts are found scattered single or in groups (very rare) throughout the epidermal region of the cnidarian body.
Absent on the basal disc.
Abundant at the oral region and on the tentacles where they form batteries of nematocysts.
Two large central nematocysts surrounded by 10-12 small nematocysts.
All these large and small nematocysts are enclosed within a single large epithelio-muscle cell.
Mechanism of Defense
The discharge or explosion of nematocysts takes place when cnidocil is stimulated by food, prey or enemy.
Both the presence of food and touch together initiates the process of explosion and not any one alone.
Response is wholly local without the involvement of nervous system.
The wall of nematocyst remains impermeable to water except during discharge.
On stimulation, the wall of the capsule suddenly increases its permeability causing rapid intake of water.
Consequently the osmotic pressure inside the capsule increases.
Operculum is forced to open up.
Coiled thread tube turns inside out and finally the whole nematocyst explodes to the outside.
Barbs and barbules present inside the shaft unfold to the outside.
Nematocyst once exploded cannot be used again.
After the explosion the cnidoblasts migrate to the gastro vascular cavity and are digested.
The exploded nematocysts are replaced within 48 hours.
Types of nematocysts
Type is constant for particular species. In Hydra, there are four basic types of nematocysts which serve various functions.
Penetrant nematocyst (Stenotele):
Very large and complex compared to other types.
Pear-shaped almost occupying the entire space of cnidoblast in which it lies.
Thread is long and hollow, coiled transversely and bearing three large barbs and three rows of small spines.
When the thread tube is discharged, it shoots out with great explosive force to pierce the victim body and injects the poisonous fluid which paralyses or kills it outright.
Hydra then seizes its prey with tentacles and draws it into its mouth.
Volvent nematocyst (Desmoneme):
Small and pear-shaped.
Contain a short, thick, spineless elastic thread tube forming single loop.
When discharged, it tightly coils around the small projections like hair or bristles of the prey and thus stopping the movement of the prey.
Stereoline glutinant nematocyst:
Oval or elongated in shape.
Do not have shaft.
Discharge a straight unarmed thread tube open at the tip.
Useful in attachment and anchorage.
Streptoline glutinant nematocyst:
Oval or cylindrical.
Their thread tube is long with a narrow shaft which forms three or four coils.
Bears a spiral row of small spines.
Mainly useful in attachment and to impede the movement of small animals.