Sponges contain spicules
made of either calcium carbonate (Calcarea) or silica (Silicea) that provide support.
They lack tissues; there are a few specialized cell
types.
- Epidermal cells form a outer layer called the epidermis.
- Porocytes allow water to enter a cavity (spongocoel) and out via the Osculum.
- Choanocytes use flagella to move water and engulf food by phagocytosis.
- Amoebocytes transport nutrients to other cells and also secrete the spicules.
Fertilization is external: 
