Proterospongia is a rare freshwater protist, a colonial member of the Choanoflagellata. It consists of a number of cells embedded in a jelly-like matrix. Interestingly, it shows a very primitive level of cell differentiation, or specialization for different roles. The flagellated cells with the collar structures move the colony through the water, while the amoeboid cells on the inside divide into new cells and so help the colony grow.
Proterospongia itself is not the ancestor of sponges. However, it serves as a useful model for what the ancestor of sponges and other metazoans may have been like. Sponges also have a low degree of cell differentiation, with collar cells and amoeboid cells arranged in a gelatinous matrix; however, sponges have other types of cells, and their choanocytes beat within canals on the inside of the sponge to pull water through the sponge -- whereas Proterospongia pulls itself through the water with its collar cells on the outside, and it lacks internal canals. Nonetheless, the similarities between Proterospongia and sponges are strong evidence for the close relationship between choanoflagellates and animals.