Phylum Cnidaria: General Characteristics and Classification | 100% Free Biology Notes

After reading this article you will learn about Phylum Cnidaria: General Characteristics and Classification.

Habit & Habitat

• Aquatic (mostly marine and few fresh water)
• Sessile or free swimming
• Solitary or colonial

Cnidaria General Characteristics

• Coelenterates are also known as Cnidaria due to presence of stinging cells called Cnidoblast
• Cnidarians composed of two layers of tissue known as ectoderm and endoderm with non cellular layer called mesogloea in between
• Epidermis is made of muscular cells, cnidoblasts, sensory cells, nerve cells and germ cells
• Gastrodermis is inner lining of the gastrovascular cavity. It is a single-layered tissue with gland cells
• and phagocytic nutritive cells
• Basic body shape of cnidarians consists of sac with gastrovascular cavity or coelenteron with a single opening
• Tentacles are finger like structures which surrounds the mouth of coelentreates
• Tentacles are used for food capture and defense
• Cnidoblasts are stinging cells with a poison filled capsule called nematocyst
• It contains a poisonous substance called hypnotoxin

Body Forms of Cnidaria

I) Polyp

• Cylindrical and sessile form
• Mouth and Tentacles directed upward
• May be Solitary or Colonial
• E.g. Hydra and Adamsia

II) Medusa

• Umbrella shaped and free swimming
• Always solitary
• Mouth and Tentacles directed downward
• E.g. Aurelia

Reproduction in Cnidaria

• Asexual by Budding
• Sexual by production of gametes
• Development is indirect
• Alternation of generation:- Its also known as metagenesis. This phenomenon shown by some cnidarians in which polyps produce medusa asexually and medusa form the polyps sexually. E.g. Obelia

Classification of cnidaria

Class 1. Hydrozoa

• Both polyp and medusa stage present
• Medusa with a velum
• Fresh and marine water
• E.g. Hydra, Obelia and physalia

Class 2. Scyphozoa

• Medusa stage dominant
• Polyp reduced or absent
• Medusa without velum
• E.g. Aurelia

Class 3. Anthozoa

• All polyps
• No medusa stage
• This class has two types of animals
  1. Sea anemones:- Skeleton absent
  2. Corals:- CaCO3 Skeleton