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