- Sponges (phylum Porifera) are the simplest multicellular animals and lack tissues and organs, existing as a porous body supported by spicules and composed of different cell types including choanocytes. - Two hypotheses for the origin of multicellularity are the syncytial ciliate hypothesis and the colonial flagellate hypothesis. - Placozoans are the sole member of phylum Placozoa, exist as two epithelial cell layers with a syncytium between, and lack organs but can glide and absorb nutrients.

1. Sponges and Placozoans

3. Dendrogram of Major Phyla Porifera Actinopoda Cnidaria Apicomplexa Ciliophora Mollusca Annelida Euglenozoa Bryozoa Platyhelminthes Rhizopoda Rotifera Granuloreticulosa Nematoda Priapulida Kinorhyncha Loricifera Phoronida Brachiopoda Echinodermata Hemichordata Chelicerata Uniramia Crustacea Protochordata Vertebrata

32. Cladogram of Sponge Classes