Sponges represent the cellular level of organization. Some are asymmetrical, but most are radially_symmetrical. They have motile_larva and are sessile as adults. Most are marine and all are aquatic. Sponges are little more than loosely organized groups of cells.
They can reproduce both asexually as well as sexually. Asexual reproduction is accomplished by budding or the production of gemmules. Sexual reproduction involves the production of a ciliated_larva.
Their skeleton made of spicules, which can be composed of silicon, spongin protein, or calcium.
Specialized cells include:
(1) Pinacocytes: Contractile epidermal cells.
(2) Choanocytes: are called collar_cells; these cells are flagellated feeding cells with a network of microvilli. Choanocytes line the canals that bring water into the spongeocoel. They are used for filter-feeding.
(3) Amoebacytes: Groups of cells with a variety of functions such as producing spicules, fibrils and reproductive cells.
Specialized structures include:
(1) Ostium (pl: ostia): These are the small pores that cover the surface of the sponge. Water enters the body through these pores.
(2) Osculum: The osculum is the large top surface opening, through which water exits the spongeocoel.
(3) Spongeocoel: The spongocoel is the central cavity of a sponge. Think of a vase - the hollow center that holds water would be the equivalent of the spongocoel. A spongocoel is not a coelom_cavity, just a hollow area.
(4) Radial and incurrent_canals: These are lined with choanocytes. Incurrent canals bring water from the outer surface (via dermal_pores) into the wall of the sponge. Radial canals connect the incurrent canals with the spongocoel. As water passes through the walls of incurrent canals to enter the radial canals, choanocytes within the walls filter-feed.