You, too, can learn about the basis of metazoan evolution. PZ Myers on Pharyngula has written,
“What were the key innovations that led to the evolution of multicellularity and what were their precursors in the single-celled microbial life that existed before the metazoa?”
It seems that some one-celled animals with flagella, called choanoflagellates, became involved in sponge development. They resemble the choanocytes of sponges, which line the sponges’ inner passages and fan their flagellae to move sea water and food particles. With their stickiness or adhesion molecules, choanocytes aided the eventual transition to multi-cellular organisms, or metazoans.
Read the whole article for a better idea of what was going on before the development of hard-shelled animals in the Cambrian: “The choanoflagellate genome and metazoan evolution.”