Mostly external but in few species like hydra have internal fertilization 

Introduction:
Coelenterata is a phylum of aquatic animals that includes jellyfish, sea anemones, and corals. Fertilization is the process of fusion of male and female gametes, leading to the formation of a zygote. There are two types of fertilization - external and internal. In this article, we will discuss the type of fertilization present in coelenterata.

External Fertilization:
External fertilization is the type of fertilization that occurs outside the body of an organism. In coelenterata, external fertilization takes place. During spawning, both male and female gametes are released into the water, and they fuse together to form a zygote. The zygote develops into a planula larva, which then settles on a substrate and forms a polyp.

Advantages of External Fertilization:
There are several advantages of external fertilization in coelenterata, which include:

- Increased genetic diversity: External fertilization promotes genetic diversity as it allows gametes from different individuals to fuse and form a zygote with a unique genetic makeup.

- Reduced competition: External fertilization reduces competition among individuals for mating partners as both males and females release their gametes into the water.

- Increased chance of survival: The large number of gametes released during external fertilization increases the chances of survival of the zygote as it has a better chance of encountering a suitable substrate for settlement.

Conclusion:
In conclusion, coelenterata exhibit external fertilization where both male and female gametes are released into the water, and they fuse together to form a zygote. This type of fertilization promotes genetic diversity, reduces competition for mating partners, and increases the chance of survival of the zygote.