Cnidarians

Cnidarians were formerly known as coelenterates. They include hydroids (hydrozoans or water animals), sea anemones and corals (anthozoans or flower animals), and jellyfishes (scyphozoans or bowl animals). They vary in form, color, and shape.

The two body types of cnidarians are the polyploid and the medusoil. The polyploid body type is common among corals and anemones. This type has a mouth and tentacles that are directed upward. The bottom part of the body is attached to a substrate. The medusoid body type, on the other hand, has a mouth and tentacles that are directed downward. This body orientation is observed in motile kinds of cnidarians like jellyfishes.

All species have tentacles with stinging cells. These cells found at the tip of the tentacles are used to capture prey. Cnidarians are carnivores that mostly feed on zooplanktons. Most cnidarians have stingers called nematocysts, which secret a toxic substance that can kill their prey. This substance can also be deadly to humans.

Coral colonies are formed by small marine invertebrates called coral polyps. In their early stage of development, polyps are free-swimming larva that eventually become sessile animals. They produce asexually by budding and sexually by releasing sperm and eggs in water. Millions of coral polyps form the coral reef, which are built when hard corals form numerous colonies.