Cnidaria

Cnidaria is a diverse phylum of simple, predominantly marine invertebrates, including jellyfish, corals, sea anemones, and hydras. These animals are known for their distinctive body plan, which features a central body cavity and radial symmetry. The name "Cnidaria" comes from the specialized cells called cnidocytes, which contain stinging structures known as nematocysts, used for capturing prey and defence.

1.0Introduction

General features of the phylum Cnidaria

• All aquatic, some freshwater, mostly marine. 
• Solitary or colonial, Sedentary or free swimming.

• Symmetry radial or biradial about a longitudinal oral aboral axis.
• Body organization of cell-tissue grade. 
• Exoskeleton chitinous or calcareous. 

• Body wall diploblastic with two germ layers- outer epidermis and inner gastrodermis, with gelatinous acellular mesoglea in between.

• Two types of individuals occur: attached polyps and free-swimming medusae. 
• Slender tentacles often encircle the mouth of the polyp and the bell margin of medusae. 

• A single internal cavity lined with gastrodermis- called gastrovascular cavity or coelenteron is present in which the mouth opens. Anus absent. 10. One or both body layers have cnidoblast cells, with a peculiar stinging cell organelle- nematocysts- for adhesion, food capture, offence and defence.
• Asexual reproduction by budding or fission. Sexual reproduction by sperm and ova. Sexual forms are monoecious or dioecious. 
• Development includes a free-swimming ciliated planula larva. 
• Life history illustrates a regular alternation between the asexual polypoid stage and a sexual medusoid stage- which is called metagenesis. True alternation of generations absent. 

2.0Classification of Phylum Cnidaria 

Class: Hydrozoa (Greek: Hydro, Water Serpent)

• Individuals are either exclusively polypoid or medusoid or constituted by both forms in the life cycle. 
• Medusa with true velum  

• Mesoglea acellular. 
• Cnidocytes are confined to the epidermal layer. 
• Coelenteron is undivided and without stomatoeum. 
• Gonads are usually ectodermal in origin, or gametes do not escape through the coelenteron and mouth if endodermal. 

Examples: 

Class: Scyphozoa (Greek: Skyphos, Cup) 

• The Medusoid form is dominant in the life cycle; the polypoid form is insignificant. 
• Mesoglea is cellular. 
• Some cnidocytes are endodermal. 
• Velum is absent. 
• Gonads are endodermal. 

Example: 

Class : Cubozoa (Greek: Cubo, cube) 

• Medusoid cnidarians with cubical bells. 
• The body is distinctly flattened to form four sides. 
• Bell margin simple. 
• Presence of velum along the margin of the medusa. 
• There are four tentacles or tentacle clusters at the four corners of the margin. 
• The body wall possesses a type of nematocyst called stenotale. 

Examples: 

Class Anthozoa (Greek: Anthos, Flower)

• It only exists in polypoid form. 
• Mesoglea cellular. 
• Some cnidocytes are endodermal. 
• The Stomodaeum was strongly developed. 
• There are mesenteries between the stomodaeum and the body wall. 
• Gonads are endodermal. 

Subclass Octocorallia/Alcyonaria

• Polyp with eight tentacles and eight septa (mesenteries). 
• Mesenteries are complete. 
• Tentacles are pinnately branched. 
• Only one ventral siphonoglyph is present. 
• Almost entirely colonial, usually connected by coenenchyme. 

Examples:

Subclass Hexacorallia / Zoantharia

• Polyps with more than eight tentacles and septa, typically in cycles of 12. 
• Tentacles are hollow and un-branched. 
• Usually, two siphonoglyphs are present. 
• Solitary or colonial. 
• Examples: Sea-anemones (Adamsia, Edivardsia, Metridium)

3.0Obelia Colony

• It is a branched, fixed colony. 
• Some horizontal branches anchoring the colony on some support are called Hydrorhiza, while others are vertical and called Hydrocaulus. 
• Each branch consists of coenosarcs made of two cell layers enclosing the coelenteron and surrounded by a thin transparent perisarc. 
• The colony bears three types of zooids. 
• a. Polyps or gastro zooids (vegetative zooids). Barrel-shaped and responsible for the nutrition of the colony. The perisarc enclosing the polyp is termed hydro theca. 
• b. Blastostyles or gonozooids. Club-shaped zooids bearing the medusae buds. 
• c. Medusae buds. Umbrella-like reproductive zooids bearing gonads, enclosed in a gonotheca. 

4.0Life Cycle of Obelia 

• Medusae buds are unisexual and free-living at maturity. 
• One medusa bears four testes or four ovaries close to the four radial canals. When the gonads are ripe, ectodermal covering ruptures, and the germ cells are shed in water. 
• Fertilization occurs in the seawater where the germ cells are set free, or the spermatozoa may be carried by water currents to the female medusae and fertilize the ova in situ. 
• The zygote forms after fertilization and immediately undergoes cleavage. The cleavage is holoblastic, and a blastula is formed. 
• By invagination, the blastula is converted into an oval, ciliated planula larva. 
• The planula consists of an outer layer of ciliated ectoderm and an inner mass of endoderm cells enclosing a space, the rudiment of the coelenteron. 
• The planula swims freely briefly, settling down on some submerged substratum by one end. 
• The proximal end gradually narrows down, and a disc appears for attachment. 
• The distal end expands, developing a manubrium and a circle of tentacles, which turns into a hydraulic or simple polyp. 
• The hydrula sends out lateral buds, which are converted into a complex obelia colony by repeating this process

5.0Metagenesis

• Alternation of generations refers to a biological process in which an organism undergoes a regular alternation between a diploid asexual phase and a haploid sexual phase during its life cycle. 
• This type of generation alternation is commonly observed in plants such as mosses and ferns.
• Obelia's life cycle includes two clearly defined phases: a fixed polypoid diploid phase (hydroid colony) and an aquatic medusoid diploid phase. 
• The hydroid colony is sexless, bears no gonads, and develops through an asexual process, i.e., repeated budding of the hydra. 
• But the medusae buds, some of the zooids of the colony, develop gonads, and a new Obelia colony arises from their fertilized egg. 
• The asexual generation is dependent on and is alternated by the sexual generation. This fact has given rise to the alternation of generation, although it is not the true generation. 
• So, a separate name has been given, which is called metagenesis. 

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