Cnidarians are important group in animal kingdom. Cnidaria also called vicious specie includes hydra, jellyfish, coral and sea anemones. They are soft and squishy in nature but all have capability to sting their prey or for defense. They are carnivorous. Coral and sea pens produce calcium carbonate to shape themselves as a rock for their protection. There are two body forms of aquatic invertebrate phylum cnidaria called polyp and medusa. The main difference between them is, the polyps are cylindrical and usually fixed form of cnidarians while medusas are free motile form of cnidaria and usually an umbrella like.
Polyps are barrel- shaped and immobile specie of Cnidaria. They majorly involve hydra, coral and sea pen as cnidarians specie. It represents the asexual phase of hydra and scyphozoa. Coral and sea anemone are pure polyps. Polyps attach itself to ground and on the other top side are its mouth and tentacles. It has an elongated axis. Polyps can either be solitary or in groups. the groups can be attach to ground or each other. The hold fast called paddle disc holds the polyp in a position. Mesoglea is preset between the single layered ectoderm and endoderm but it is not that much thick. It is considered to be living fossils as even after the mass extinction event it is still surviving since more than half a billion years. They are dependent on others and reproduce through asexual budding.
The umbrella like motile medusa is free to move. It represents the sexual phase. It is retrograde form of polyp. Medusa is flattened sac-like floating with its mouth facing down and tentacles above. The upper body of medusa is called exumbrella while the lower part is called subumbrella. In Portuguese man-of-war the upper part called exumbrella are slightly convex and sometimes round or egg-shaped while subumbrella is concave. There are three type of tentacles. Every set of tentacle play different role, like one set is responsible for reproduction, the other is responsible for protection while one set is for feeding. The example of medusa is jelly fish. Between the single layered ectoderm and endoderm is a jelly like substance called Mesoglea which makes the body wall thick. Medusa also have a air cavity called pneumatophore, this is responsible to keep the specie floating in the water column. Medusa can also be solitary or in colonies. If in colony it has same pneumatophore as other.
- Polyps are anchored, rigid barely moving specie found near the tidemark, they are attached to rocks while medusas are freebie floating, found in open water.
- Polyp has a tubular body with a protracted stalk while medusa has saucer shaped body with shortened trunk for example jelly fish.
- In polyp the body is attached below thus the manubrium is skyward while in medusa base is above so that manubrium hover earthwards.
- A fine parchment called vellum is absent in a polyp while it is present in a medusa.
- There is no sense organ to feel the environment around in a polyp while at the base is eight adradial tentacles that give sensation to medusa.
- A straightforward gastro vascular cavity is found in the polyp without any radial or circular canals while in medusa gastro vascular represents stomach, four radial canals and one circular canal is preset.
- There is no gonad in a polyp while in medusa on radial canals, there are four gonads.
- Polyp reproduces abiogenetically while medusa replicates sexually.