Which group of animals includes hydras and jellyfish?

The group of animals that includes hydras and jellyfish is Cnidaria. This phylum is characterized by the presence of specialized cells known as cnidocytes, which contain stinging structures called nematocysts. These cells are used for capturing prey and for defense. Members of the Cnidaria phylum exhibit a simple body plan that includes a central gastrovascular cavity and typically have two main forms: the polyp (as seen in hydras) and the medusa (as seen in jellyfish).

Cnidarians are also known for their radial symmetry, which allows them to interact with their environment from multiple directions and is another defining feature of this phylum. Their simple body structure, presence of stinging cells, and aquatic habitat make them a distinct and important group within the animal kingdom. The other options represent different phyla with their own unique characteristics and do not include hydras or jellyfish.