So we have a multicellular organism with specialised cells, epithelial cells, feeding cells, transporting cells, skeleton forming cells, muscle cells. The only major group not represented is the nerve cell. Behaviour is apparently a local affair, with individual cells responding as they are stimulated. Sponges sometimes also have sexual reproduction. Certain mesenchme cells divide then enlarge to form eggs or sperm.
Some sponges are hermaphrodite, but others are unisex.
In either case sperm are shed into the excurrent to reach eggs which are retained. The fertilised egg divides, divides again into 4 and more divisions form a hollow ball of ciliated cells one layer thick. The baby sponge is cast into the world at this stage. It swims around, and if not eaten settles down, and becomes a young sponge by a rather complicated manoeuvre. Sponges are of interest to us because they show, in perhaps its simplest form, the cellular level of organisation, at least as far as morphology goes .