The first eukaryotes evolved somewhere around 1.5 billion years ago. The first animals evolved somewhere around one billion years and the most primitive today are the sponges, in the phylum porifera. The sponges are characterized by their colonial nature. They have no tissues, no organs, and are a loosely organized group of cells. Each cell is responsible for its own nutrition, and cannot eat anything larger than a bacteria.
Sponges have irregular symmetry or no symmetry.
Water filters into a sponge through tiny pores where flagellated cells called choanocytes filter out food. These are the workhorses of the sponges. The second important cell in sponges are called amoeboid cells. Spicules are made of silica or calcium carbonate. Sponges inhabit every environment, from marine-tropical to polar.