Some species also form internal buds, called gemmules, which can survive extremely unfavorable conditions that cause the rest of the sponge to die. Sexual reproduction takes place in the mesohyl. Male gametes are released into the water by a sponge and taken into the pore systems of its neighbors in the same way as food items. The larvae may settle directly and transform into adult sponges, or they may be planktonic for a time. Adult sponges are always sessile.
Sponges fall into three main groups according to how their bodies are organized.
The simplest sponges are the asconoid sponges. These are shaped like a simple tube perforated by pores. There is a single opening to the outside, the osculum. The next-most complicated group is the syconoids . These tend to be larger than asconoids.