Nematocysts (Gr. Nema = thread + kystis = bladder) are specialized stinging cells specific to Coelenterates that act as organs of offence and defense. They are also called Cnidae and hence the coelenterates are also called Cnidarians. The cells that produce nematocysts are called nematoblasts.
STRUCTURE
Nematocysts are cell organelles found in specialized cells called cnidocytes or cnidoblasts, which are modified epidermal interstitial cells. A Cnidoblast is an oval or rounded cell with a conspicuous nucleus lying on basal side. Inside the cnidoblast is present an oval or pyriform sac or bladder that is filled with a toxin called hypnotoxin, which is a mixture of proteins and phenols. The outer end of the capsule is invaginated into a long, hollow and tubular filament or thread tube, coiled like a watch-spring inside the sac itself. The base of the thread tube is swollen to form a butt, which carries three large spines, called barbs or stylets and three spiral rows of minute spines. The bladder is covered on the top by a lid or operculum, near which lies a hair-like process, the cnidocil that acts like a trigger for the discharge of the thread tube. Cytoplasm of cnidoblast may contain contractile muscle fibrils and a restraining thread at the base, called lasso, which prevents the nematocyst from being thrown out of cell. Otherwise cnidoblast has cell organelles similar to a normal cell and at base it is attached to an epithelio-muscle cell.
Nematocysts may occur scattered throughout the body surface, except the basal disc. They are abundant on the tentacles and hypostome. On tentacles they form “battery of nematocysts” which are used for paralyzing the prey. A battery consists of one or two large central nematocysts surrounded by 10-12 smaller ones.
MECHANISM OF DISCHARGE
The discharge of nematocyst thread tube from the cnidocyte takes place due to mechanical or chemical stimuli received by the cnidocil. In a resting stage the capsule wall is not permeable to water and there is very high osmotic pressure since the hypnotoxin inside is hypertonic to external water. As the cnidocil is triggered, the permeability of the capsule wall increases causing water to rush inside the capsule, resulting in an increased hydrostatic pressure. Owing to high hydrostatic pressure the opercular lid opens and the thread tube inverts inside out with great force and penetrates deep in the tissue of the victim injecting toxic hypnotoxin. The capsule shrinks and the thread tube once discharged cannot be withdrawn. The discharged nematoblasts are replaced by the new one. Nematocysts do not require nervous stimuli for activation, although in fully fed animals, nematocysts are not discharged when they come in contact with food.
TYPES OF NEMATOCYSTS
About 30 different types of nematocysts are found in different groups of cnidarians.
Penetrants: Penetrants are of two types, stenotele found in Hydra and euryteles found in jelly fishes. They are the largest (16 microns in diameter) and most complex nematocysts. It is pear-shaped and its thread tube is long and hollow, coiled transversely and bearing three large stylets and three rows of small spines at its stout base. Thread tube is open at the apex and when discharged, it darts out with such explosive force that it pierces the body of prey and injects the hypnotoxin that paralyzes the victim or kills it outright.
Volvents: Volvents or desmonemes are a small (9 microns long) and pear-shaped nematocysts containing a short, thick, spineless, smooth and elastic thread tube that is closed at the apical end. When discharged, the thread tube tightly coils around small projections such as hairs or bristles of the prey and holds it in place.
Spirocysts are found in sea anemones in which thread tube is spirally coiled around the capsule in resting stage.
Ptychocysts are also found in anthozoans. The thread tube bears fine tubules that form adhesive net on release.
Glutinants secrete a sticky substance that helps in fastening the tentacles to solid objects and hence help in locomotion and food capture. They also inject hypnotoxin. They lack a butt and their thread tube is of same diameter throughout.
Stereoline glutinants: They are small oval or elongated glutinants also called atrichousisorhizas because they do not possess spines and butt. They discharge a straight and unarmed thread tube open at the tip.
Streptoline glutinants: Also called holotrichousisorhiza, they are oval or cylindrical in shape and the thread tube is long with a narrow butt. The thread tube is pointed and opens at the tip and bears a spiral row of small spines.