Define nematocyst, polyp, medusa, mesogloea, planula and coral

animal-kingdom
cbse

#1

Nematocysts: It is stinging organ of coelenterates. It is produced inside a stinging cell called nematoblast or cnidoblast which consists hypnotoxic fluid and a long coiled tube for injecting the fluid into prey or predator. It is used as an organ of offence and defence.
Polyp : It is a free living sessile or mobile zooid or individual of a simple or colonical coelenterate. It is cylindrical in outline with a terminal mouth surrounded by a ring of tentacles, e.g., Hydra, Physalia, coral.
Medusa : It is a bell-like or umbrella-shaped free floating or swimming individual (zooid) of coelenterates.
Mesogloea : It is a non-cellular gelatinous middle layer found between ectoderm and endoderm as in coelenterates.
Planula : This is a free swimming young ovoid ciliated larva of coelenterates. It gives rise to the adult individual.
Coral : It belongs to actinozoa-coelenterate which secretes calcareous skeleton. Coral is commonly a colonial polyp but occasionally solitary corals is also niet. Coral skeletons pile up and form coral reefs. Great barrier reef of north-east coast of Australia is a coral reef. At places coral reefs form islands.