General characteristics of phylum Coelenterata/Cnidaria

  • Coelenterates are the first metazoan also called cnidarians (stinging animals).
  • They have one continuous space or gastrovascular cavity called coelenteron.
  • Coelenterates are aquatic (may be marine or fresh water).
  • They are diploblastic animals and have a radially symmetrical body.
  • Cnidarian are at tissue level of body organization and have a division of labor.
  1. Body wall:
  • Body wall has outer ectoderm, inner endoderm or gastroderm with a non-cellular mesoglea in between the two.
  • Different types of cells like endothelio-muscle cells, glandulo-muscle cells, interstitial cells, cnidoblasts or nematocysts, germ cells, sensory cells and nerve cells are present in the ectodermal layer.
  • Gastroderm or endoderm contains five types of cells namely; endothelio-muscle cells or nutritive cells, endothelio-gland cells, interstitial cells, sensory cells and nerve cells.
  • The interstitial cells are capable of developing into any of the other kind of cells like reproductive , glandular, stinging cells etc. as required. They are thus totipotent, i.e. they renew all the cells of the animal.
  1. Mode of nutrition:
  • Coelenterates have a blind sac body plan. They are acoelomate (coelom is absent).
  • Alimentary canal is incomplete, i.e. the gut cavity has a single opening called mouth.
  • Mouth serves for ingestion and egestion and the anal opening is absent.
  • They are carnivorous and can digest all food but starch.
  • Digestion is unique with extracellular digestion followed by intracellular digestion.
  1. Body structure:
  • Body is short and slender having tentacles encircling the mouth in one or two whorls.
  • Tentacles serve for food-intaking, capturing the prey and defence.
  • Tentacles in Hydra are hollow while solid in Obelia.
  • They show polymorphism, i.e. two distinct forms (polyp and medusa) of individuals are seen.
  1. Other body systems:
  • Nervous system is in diffused state with network of nerve fibers in the body walls and the tentacles. The brain is absent.
  • Circulatory, respiratory and excretory systems are absent. Gaseous exchange and excretion occur through general body surface. They are ammonotelic.
  • Locomotion and movement occurs with the help of smooth muscle fibrils in the epithelia or ectoderm.
  • Specialized stinging cells called cnidoblasts are present which secrete hypnotoxin (poison).
  • Sensory organs may be single or complicated, some with eye spots called statocysts.
  1. Reproduction:
  • Reproduction is either asexual (by external budding) or sexual (by the formation of ova and sperm).
  • Planula is a common larva but Ephyra larva are present in some animals.
  • Their life cycle shows alternation of generation, in which a sexual free swimming medusoid generation alternates with an asexual sessile, usually colonial, polypoid generation.
  • On the basis of dominance of zooids, coelenterates are divided into following three classes.

Class: Hydrozoa

  • They are mostly marine but few are fresh water species.
  • Body wall consists of two layers of cells, i.e diploblastic.
  • Asexual polyp stage and sexual medusa stage are present
  • Medusa stage is with velum.
  • Polyp is the dominant stage.
  • Mesoglea is simple and acellular and the gonads are ectodermal in origin.
  • They have statocysts to maintain equilibrium.
  • e.g. Hydra, Obelia, Physalia etc.

Class: Scyphozoa

  • They are exclusively marine and triploblastic animals.
  • They are free swimming and are solitary.
  • Mesoglea is cellular and thick (jelly like) hence commonly called jelly fish.
  • Polyp stage is rudimentary or absent.
  • Medusa is domoinant, bell or umbrella shaped and lacks a velum.
  • Larva is called Ephyra.
  • Gonads are endodermal in origin.
  • They have tentaculocytes to maintain equilibrium.
  • e.g. Aurelia, Pilema etc.

Class: Anthozoa

  • They are exclusively marine and are colonial forms.
  • Body is triploblastic with mesoglea consisting of fibrous connective tissue and amoeboid cells.
  • Polyp stage is present but medusa is absent.
  • Gonads are endodermal in origin.
  • Reef forming true stony corals belong to this class.
  • e.g. Tubipora, Alcyonium, Heliopora, Gorgeonia, Pennatula etc.

General characteristics of phylum Coelenterata/Cnidaria