What are coral polyps? Write in brief.

Hint: Corals belong to the class Anthozoa of phylum Cnidaria. They are invertebrates and are exclusively marine. They are the important reef builders and inhabit the oceans. They build reefs which secrete calcium carbonate to form a hard skeleton.

Complete answer:
A coral or a coral group is a colony of identical polyps. Coral are sessile organisms and are situated at the base of the ocean or stuck to rocks. The coral colonies are formed by several individual polyps and they are genetically similar organisms that make up the colony.
The polyp is a sac like animal and secretes an exoskeleton near the base. The body is attached to a hard surface in a cup like depression which is known as the corallites. The polyps are two layered body and outer layer is known as the ectoderm and inner layer is the endoderm. A gelatinous substance is found in between the two layers and is known as mesoglea.
The upper end of the sac, an opening or the mouth is present which is encircled by a number of tentacles. The tentacles help in capturing food and tactile senses. The polyps are interconnected with other polyps by a complex and well developed system of gastrovascular canals. This allows the nutrient sharing in the colony of polyps.
The individual polyps of the colony are connected with each other through a living tissue known as the coenosarc. The polyps over several generations, secrete a calcium carbonate secretion which produces skeleton that is the important characteristic of the coral reefs. The polyp secretion protects the organism and strengthens the colony. The secretion is released by the polyps and through the coenosarc.
The colonies formed by the polyps can be either soft corals or hard corals. The soft corals do not have solid exoskeleton but the tissues contain small supportive elements which are known as sclerites. The hard corals are also called stony corals and formed by secretions of calcite.

Note: Corals are made up of microscopic organisms known as the polyps. The polyps live in colonies which are made by individual polyps that are interconnected with each other. Polyps are sac like structure and bear a single mouth with tentacles. They secrete a secretion of calcium carbonate.