Invertebrates show increasing complexity from sponges to echinoderms. Sponges are multicellular but lack true tissues, while echinoderms have water vascular systems and five-part radial symmetry. Key invertebrate phyla include porifera, cnidaria, platyhelminthes, nematoda, mollusca, annelida, arthropoda, and echinodermata. Arthropods are the most diverse and include insects, arachnids, and crustaceans. Chordates that lack backbones, such as tunicates and lancelets, are also invertebrates.