Phylum Coelenterata or Cnidaria- Important Characteristics

Phylum Coelenterata or Cnidaria:

Cnidarians are the first true metazoans that have attained tissue grade of construction.
Cnidaria is a phylum of diploblastic, acoelomatic animals that possess stinging cells and a blind sac gastrovascular cavity.
The phylum evolved in the Proterozoic era, more than 600 million years ago.
The term Coelenterata was coined by Leuckart (1847).

Characteristics of Phylum Coelenterata or Cnidaria:

Coelenterates are mostly marine except for Hydra which is a freshwater form. These may be solitary (Hydra) or colonial and sedentary (corals) or free-swimming (sea anemones and jellyfishes).
They are multicellular, diploblastic, radially or biradially symmetrical with tissue level of organization.
They exhibit the phenomenon of polymorphism and alternation of generation.
The body wall is two-layered, the outer epidermis and inner gastrodermis. The two enclose a gelatinous matrix called mesogloea or mesenchyme.
The tissue level of organization occurs in cnidarians.
The body cavity is called as gastrovascular cavity of the coelenteron.
These are carnivorous and holozoic in nutrition. Digestion is partly intercellular (in coelenteron) and partly extracellular (in gastrodermal cells). Coelenteron has a single opening mouth which helps in both ingestion and egestion.
Respiration and Excretion occur through the general body surface. These are ammonotelic.
The nervous system is primitive and consists of a diffused net of nerve cells.
The organs of offence and defence are nematocysts (stinging cells) or cnidoblasts.
A horny or calcareous exoskeleton as well as endoskeleton is present in many cases. Corals are specialized to secrete a calcareous exoskeleton.
They exist in two forms, Polyp and Medusa. The polyp represents the asexual phase and the medusa represents the sexual phase.
They show metagenesis i.e. Polyp reproduces asexually to produce medusa which bears reproductive organs which release reproductive cells which unite and finally give rise to Polyp.
The fertilized egg develops into a ciliated larva called Planula which gives rise to polyploid generations.

Examples- Adamsia (Sea anemone), Hydra, Porpita, Vallela, Physalia (Portuguese man-of-war), Aurelia (Jellyfish), Gorgonia (Sea fan), Pennatula (Sea pen), Meandrina (Brain coral).