Final answer:
Sponges and cnidarians are invertebrate phyla whose members remain attached to the sea floor.
Explanation:
Phylum Porifera (Sponges) and Phylum Cnidaria (Cnidarians) are two invertebrate phyla whose members remain attached to the sea floor. Sponges are non-motile as adults and attach themselves using a holdfast, while cnidarians such as jellyfish and corals also remain attached in aquatic environments.
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