They line the exterior of the sponge body wall. They are thin, leathery and tightly packed together. Choanocytes These distinctive cells line the interior body walls of sponges. These cells have a central flagellum that is surrounded by a collar of microvilli. It is their striking resemblance to the single- celled protists called choanoflagellates that make many scientists believe that choanoflagellates are the sister group to the animals .
Choanocytes are versatile cells.
Their flagella beat to create the active pumping of water through the sponge, while the collars of the choanocytes are the primary areas that nutrients are absorbed into the sponge. Furthermore, in some sponges the choanoflagellates develop into gametes. Mesenchyme Between the two layers is a thin space called mesenchyme or mesohyl. The mesenchyme consists of a proteinaceous matrix, some cells, and spicules. Archaeocytes Archaeocytes are very important to the functioning of a sponge.