Introduction to Phylum Coelenterate:

Phylum Cnidaria (called as coelenterata) has approx 9000 species. Few are located in fresh water, but greater number are marine. Coelenterates are simplest of eumetazoans. This fascinating group of animals takes its name from large cavity in body which serves as intestine. They have hydra, jelly fish, sea anemones and corals.

Characteristic Features of Coelenterates:

• All are aquatic animals
• They are radially or biradally symmetrical
• They diploblastic, with an epidermis and a gastrodermis, and a less
• cellular or non-cellular, gelatinous mesoglea in between
• They have no coelom, or separate excretory or respiratory system
• Two individual forms of coelenterate exist: polyps and medusa

Forms of Coelenterate:

There are 2 forms of individuals in Coelenterates. These forms are:

i) Polyp

ii) Medusa

Polyp is tubular, oral end being free carrying the whorl of tentacles, and aboral end joined to sub stratum by basal disc. Polyp is sedentary, benthic form.

Medusa is free swimming umbrella-like pelagic form, with mouth at the end of manubrium on subumbrellar side.

Categorization of Phylum Coelenterata (Cnidaria):

Depending primarily on whether polyp or medusa is dominant form in life cycle, coelenterates are separated into 4 classes:

• Hydrozoa
• Scyphozoan
• Cubozoa
• Anthozoa

Class Hydrozoa:

• They may be lonely or colonial forms
• There are asexual polyps and sexual medusae, although one kind may be suppressed
• Animals may be either fresh water or marine
• For eg. Hydra and Obelia

Class Scyphozoa:

• They are lonely medusa
• Polyp stage decreased or absent
• Medusae don't have velum
• All are marine
• Aurelia is typical example of this class

Class Cubozoa:

• They are solitary medusoid forms
• They have decreased polyp stage
• Medusa is square in cross section
• All are marine animals
• For e.g. Carybdea

Class Anthozoa:

• All are polyps
• They are lonely or colonial
• Gonads are gastrodermal
• All are marine
• For e.g. sea anemone

External Features of Hydra:

i) Body resembles the narrow elastic tube, closed at one end and open at other

ii) Closed end of hydra is called as foot or basal disc referred to as aboral end

iii) Has conical process, Hypostome or Oral Cone in centre of that is located the irregular or star shaped mouth.

iv) It has slender contractile and hollow tentacles between 6-10 in numbers

v) Body may get the length of 20mm or more when completely extended

vi) Generally have small hydras called as buds joined to them when food supply is in great quantity

vii) They have rounded projections on the body that are sexual organs- testes and ovaries

Adaptive Features of Hydra:

i) Foots secretes the sticky substance for Anchorage and Locomotion

ii) Interstitial cells that is chief agent in repairs, regeneration, budding and reconstructing tissues in growth Nematoblast assists in defense

iii) Hydra generally remains joined by basal disc or foot to objects under water

iv) Movement of hydra are for: capture of prey; reply to stimuli; locomotion

v) Swimming is helped by wave-like movement of tentacles

vi) Hydra is the Carnivorous animal

vii) Egestion is through mouth

viii) Gaseous exchange is performed by almost direct diffusion through cell membrane to surrounding water.

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