PHYLUM-COELENTERATA
(CNIDARIA)
There are about 9000 species of cnidarians. The name
Cnidaria (Knide — nettle or sting cells) is derived from the
stinging cell or cnidoblasts present on the ectoderm of
tentacles and body of these animals.
General Features
Some important general features of phylum—Coelenterata are
discussed below
(i) Habit and Habitat Cnidarians are exclusively Of
marine forms (Obelia, Aurelia, Physalia, Metridigm)’
but few of them are freshwater form (e.g.,
They are either sedentary (fixed) or free-swimming
forms.
(ii) Body Organisation and Body Wall They have tissue
grade organisation. Coelenterates are diploblastic. The
layers of body wall contain several types of cells, such
as interestitial cells, sensory cells, nerve cells,
epitheliomuscular cells, stinging cells (cnidoblasts),
etc.
(iii) Coelentric Cavity A coelenteron or gastrovascular
cavity is present, which is blind at one end and opens
as mouth or hypostome at other end. Mouth is used
both for ingestion and egestion.
(iv) Tentacles The mouth is surrounded by many thin
long, slender hollow structures called tentacles.
Their function is to paralyse and capture the prey
attachment, defence and offence.
(v) Skeleton A horny or calcareous exoskeleton as well as
endoskeleton is present in many cases.
The
coelenterates with exoskeleton of calcium carbonate
are called corals.
(vi) Digestion In coelenteron or gastrovascular cavity’
both intracellular and extracellular digestion occur.
(vii) Respiration and Excretion Exchange of gases and
excretion directly as all the cells are in direct
contact with water.
(viii) Sensory cells occur in the body wall such
as statocyst for balancing and ocelli for
photosensitivity.
(ix) Nervous System It is in the form of a nerve net,
i.e., the unipolar neurons are loosely arranged in
the body wall.
(x) Polymorphism Cnidarians are mainly of two types
of individuals, i.e., polyp (cup-shaped) and medusa
(umbrella-shaped). Polyp is sesile, cylindrical
structure with mouth and tentacles facing upwards,
e.g., Hydra, Adamsia, etc. Medusa is free-swimming
zooid with mouth and tentacles facing downwards,
e.g., Aurelia (jelly fish).
Medusae may form polyps after sexual reproduction
while polyps form medusae through vegetative
budding. Many variations occur in zooid forms.
This phenomenon is called which
helps in division of labour.
(xi) Reproduction Coelenterates reproduce both by
asexual and sexual methods. In many cnidarians, the
polyps usually reproduce asexually by budding to
give rise to medusae. The medusae bear gonads or
sex organs and reproduce sexually to form polyps.
(xii) Fertilisation It may be external or internal.
(xiii) Alternation of Generation The cnidarians complete
their life cycle through two phases, i.e., asexually
reproducing polyp phase and sexually reproducing
medusa phase. Those cnidarians which exist in
both forms exhibit alternation of generation
(metagenesis).
Knowledge Plus
Advancement Over Sponges The cnidarians or
coelenterates exhibit advancement over sponges, as they
possess a tissue level of organisation of the body With
well-defined layers of cells and a digestive cavity,