Class XI, BIOLOGY, "Phylum Cnidaria (Coelenterata)"

Phylum Cnidaria (Coelenterata)

MAIN CHARACTERS
HABIT AND HABITAT
They are aquatic animals, mostly marine and few fresh water forms. They are sedentary or free swimming and solitary or colonial
STRUCTURE

• The cnidaria are metazoa having the simplest type of body wall consisting of two layers. The outer epidermis and the inner gastrodermis which lines the body cavity.
• In between the two layers lies the mesogloa, non-cellular jelly secreted by them.
• Cnidarians, due to their two layers body wall are termed as diploblastic animals. All other metazons possesses a third layer called mesoderm in their body wall, laying in between the epidermis and gastrodermis (Endoderm) and are therefore called Triploblastic animals.
• They have radially symmetrical body plan organized as a hollow sac.
• The mouth is surrounded by a circle of tentacles bearing cnidoblasts stinging cells containing nematocysts.
• They have central digestive cavity connected to the outside by mouth.

STRUCTURAL TYPES
The Cnidarians are radially symmetrical and occur in two types of forms.
(a) The polyp
(b) The Medusa
(A) POLYP
The polyp like Cnidarian for example sea anemone has a cylindrical body with a mouth directed upwards and surrounded by tentacles. The basal surface of the body is attached to the substratum.
(B) MEDUSA
The medusa like Cnidarians jelly fish are umbrella like in appearance. Their oral surface, bearing the mouth is directed downwards. Whereas the aboral surface is directed upward. The medusoid Cnidarians are usually free swimming.
PROCESS OF FEEDING AND DEFENCE

• The Cnidarians feed mostly on animal diet.
• The food is digested in the gut and the waste products are expelled through the mouth.
• The Cnidarians so named, because they possess cnidoblasts bearing nematocysts which help in feeding and defence.

REPRODUCTION
The Cnidarians reproduce by asexual as well as sexual methods. Polypoid Cnidarians possess a remarkable ability to regenerate.
(A) REGENERATION
If the oral part of the body is lost. The remaining part regenerates the new mouth and the whole of tentacles.
(B) ASEXUAL REPRODUCTION
A sexual reproduction takes place by Budding.
(C) SEXUAL REPRODUCTION

• The sexual reproduction takes place through male or female gametes which are usually produced by different parents.
• The gametes develop in the interstitial cells and aggregate in gonads which are located either in the epidermis or in the gastodermis.
• The fertilized egg gives rise to “Planula Larva”

CLASSIFICATION OF CNIDARIA (COELENTERATA)
The Phylum Cnidaria is divided into three classes:
1. Hydrozoa
2. Scyphozoa
3. Anthozoa
1. HYDROZOA
As the most primitive class of the Cnidarians, Hydrozoa is thought by some evolutionists to have given rise to both other classes. They show following characteristic features:

• They are mainly marine, but some are fresh water species
• Many species have both polyp and medusa

For e.g:
Hydra, Obelia and Physalia
2. SCYPHOZOA

• Most of animals of this class are commonly called “Jelly Fish”.
• They are semitransparent and are of various colours.
• Most are of marine habitat.

For e.g:
Aurelia and Cya