sentence: Some species live singly, others live in colonies.
sentences_before: The 'Hydrozoa' are a class of the phylum Cnidaria. They are small predatory animals which mostly live in the sea. Their basic life form is the polyp.
sentences_after: Their classification and evolutionary relationships are still under discussion. The hydroids may be best understood by taking some examples.
concept_name: specie
headings: Hydrozoa
categories: Cnidarians