Cnidarians are invertebrates such as jellyfish and corals. They belong to the phylum Cnidaria. All cnidarians are aquatic. Most of them live in the ocean.

However, both cnidarians and ctenophores have a type of muscle that, in more complex animals, arises from the middle cell layer. As a result, some recent text books classify ctenophores as triploblastic, and it has been suggested that cnidarians evolved from triploblastic ancestors.

jellyfish
Integrated Taxonomic Information System – Report

Reproduction of Cnidarians In general, polyps primarily reproduce asexually by budding, however, some produce gametes (eggs and sperm) and reproduce sexually. Medusae usually reproduce sexually using eggs and sperm. The planula then develops into a polyp that can reproduce either sexually or asexually.

Cnidarians don’t have lungs, and even though they live in aquatic environments they don’t have gills either. Instead of breathing, gas exchange in Cnidarians occurs through direct diffusion.

All cnidarians are carnivores. Most use their cnidae and associated toxin to capture food, although none is known actually to pursue prey. Sessile polyps depend for food on organisms that come into contact with their tentacles.

phylum Cnidaria
The Scyphozoa are an exclusively marine class of the phylum Cnidaria, referred to as the true jellyfish (or “true jellies”). It may include the extinct fossil group the Conulariida, whose affinities are uncertain and widely debated.

Class Scyphozoa, an exclusively marine class of animals with about 200 known species, includes all the jellies. The defining characteristic of this class is that the medusa is the prominent stage in the life cycle, although there is a polyp stage present.