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Submitted by Mousumi Sepai, Last Modified on 2020-02-02

Cnidocyte is a unique and define in features of Cnidarians. It is a combine sensory effector cell.

Structure of cnidocyte:-

  1. Each cnidocyte houses a cnida which is a fluid filled membrane capsule containing a long tubular invagination of the capsule wall.
  2. When the cnidocyte receives appropriate stimulation, the tubule averts explosively to the exterior.
  3. Depending on the type of cnida, the everted tubule may sting or paralize the prey by penetrating the integument and release in toxin, adhere by sticking to the surface of an animal or substrate or perform other tasks.

  4. Cnidocyte often bear a single sensory cilia and thus appear to be independent sensory effector cells.
  5. Cnidocytes typically occur throughout epidermis, but are in high density on the tentacles and other preycathing or defensive structures on or near the mouth.
  6. Many Cnidarians also have cnidocyte in localised part of the gastrodermis.

Types of cnidocytes:-
(i) Nematocysts
(ii) Spirocysts
(iii) Ptychocysts

(i) Nematocysts :-

Nematocysts are thick wall cnidae which occur in all groups of Cnidarians. They often spines or berbs on the surface of the invaded. The firing site may be covered by a hinged lid called operculum[in scyphozoans and Hydrozoan] or by three optical flaps in anthozoans.

The sensory cilium of the nematocysts is mechanoreceptive may be, non-motile and stiff and is called a Cnidocil[Scyphozoan and hydrozoan] or it may be motile and called a ciliary cone[Anthozoan]. All stinging and toxic Cnidae are nematocysts cells housing such Cnidae are called nematocysts.

(ii) Spirocyst:-

Spirocyst is thin wall cnidae in which the undischarged tubule in coiled like a spring. The discharged spirocysts tubule lacks burbs or spines, but they mynetute sticky threads radiate from the wall of the tubule. Cells housing such cnidae are called spirocytes. Spirocytes lacks a sensory cilium, Spirocytes occurs only on anthozoans.

(iii) Ptychocyst:-

Ptychocysts are similar to the Spirocysts and releases and adhesive tubule. The tubule is the without any radiating sticky threads. When it undischarged, the tubule is folded in a compact zigzag fashion. Ptychocyte bears a motile cilium, Ptychocytes occur in the epidermis of tube anemones(Anthozoa).

Mechanism of firing:-

  1. The mechanism of nidae discharge has been determined largely from research on hydrozoan nematocyst.
  2. The nematocyst wall is thickened and stiffened by a layer of collagen.
  3. Prior to discharge the hollow inverted tubule lies coiled and fold it within the capsule and is surrounded by fluid that contains a high concentration of ions, amino acids and proteins.

  4. If the nematocysts is venomous some of those proteins are toxins, including some that interfere with sodium pumps.
  5. Many of the ions specially calcium ions are complex with venoms and other proteins to form large aggregates that create little osmotic pressure.
  6. When the nematocyst is appropriately stimulated, these ions dissociate from their macromolecules to create a high intracapsular osmotic concentration.
  7. These causes water from the cytoplasm to rush into the capsule, pressuring it and everting the tubule foreibely and rapidly.
  8. Presumably the thick wall of the nematocyst prevence it from sualling appreciably during the water in a rush.
  9. The tubule may be fully everted in less than 2 milliseconds, but may recur as long as a few seconds, depending on the type of Nematocyst.
  10. Once the nematocyst is discharged, the nematocyte degenerates and is replaced by a newly differentiated cell.
  11. Above 25% of the nematocyst of Hydra littoralis are lost from the tentacles in the process of eating.
  12. The discharge nematocyst are replaced within 48 hours.
  13. Specific mechanical and chemical stimulating can cause firing. Mechanical stimulation includes vibrations at particular frequencies and chemical stimuli includes certain amino acids and sugars. Associated with mucus.

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