Sea sponges reproduce asexually by budding and sexually by releasing male gametes into the water. These gametes are taken in by other sponges, which then produce blastulas that are also released into the water. Budding can be external or internal. Internal budding is reserved for harsh conditions, in which the internal offspring is protected as the parent sponge dies around it.
Sponges can have separate or both sexes, depending on the species. Sponges take in sperm from the water just as they do food, using their special collar cells to draw in water and particles. When sperm is received, the collar cells lose their collars and change to an amoeba-like form. These cells then move to carry the sperm to the sponge's egg cells.
Sponges have the simplest organization of any animal, possessing specialized cells but no tissues or organs. Every sponge has a system of pores and canals through which water passes, driven by the flagellae of the cells lining these channels. These cells are supported by a skeleton of collagen reinforced by spicules, which are composed of silicon or calcium compounds, depending on the species. The individual cells envelop and digest food items rather than carrying them to any central digestive region.