These sea creatures are harmless despite their huge size. Watch the video about giant clams dwelling on the bottom of the Pacific Ocean. It is noteworthy that the phylum Mollusca is very diverse. The largest species of invertebrate animals, the giant squid and the colossal squid, belong to Mollusca, too. The same can be said about octopuses which, together with squids, belong to the class Cephalopoda.