Updated by: Arabinda Naik
There are about 9000 species of cnidarians. The name Cnidaria is derived from the stinging cell or cnidoblasts present on the ectoderm of tentacles and body of these animals.
General Features
Habit and Habitat Cnidarians are marine form (Obelia, Aurelia, Physalia. Merridium) and freshwater form, while is Hydra. They are either sedentary (fixed) Or free-swimming forms.
Body Organisation and Body Wall They have tissue grade organisation. Coelenterates are diploblastic. The layers of body wall contain several types of cells, such as interstitial cells, sensory cells, nerve cells, epitheliomuscular cells, stinging cells (cnidoblasts), etc.
Coelenteric Cavity A coelenteron or gastro- vascular cavity is present, which is blind at One end and opens as mouth or hypostome at another end. Mouth is used both for ingestion and egestion.
Tentacles The mouth is surrounded by many thin, long, slender hollow Structures called tentacles. Their function is to paralyse and capture the prey, attachment, defence, and offence.
Skeleton A horny or calcareous exoskeleton as well as endoskeleton is present in many cases. The coelenterate with exoskeleton of calcium carbonate are called corals.
Digestion In coelenteron or gastrovascular cavity, both intracellular and digestion occurs.
Respiration and Excretion Exchange of gases and occurs directly as all Cells are in direct contact with water.
Sensation Sensory cells occur in the body wall such as statocyst for balancing and ocelli for photosensitivity.
Nervous System It is in the form of nerve net, i.e., the unipolar neurons are loosely arranged in the body wall.
Polymorphism Cnidarians are mainly of two types of individuals polyp and medusa. Polyp is sessile, cylindrical structure with mouth and tentacles facing upwards Medusa is free swimming zooid with mouth and tentacles facing downwards, Aurelia or jelly fish. Medusac may form polyps after sexual reproduction while polyps form medusae through vegetative budding. Many variations occur in zooid forms. This phenomenon is called polymorphism, which helps in division of labour.
Reproduction Coelenterates reproduce both by asexual and sexual methods. In many cnidarians, the polyps usually reproduce asexually by budding to give rise medusae. The medusae bear gonads or sex organs and reproduce sexually to form polyps.
Fertilisation It may be external or internal.
Alternation of Generation The cnidarians complete their life cycle through two phases, i.e., asexually reproducing polyp phase and sexually reproducing medusa phase. Those cnidarians which exist in both forms exhibit alternation of generation (metagenesis).