Let me share with you a difference between ostia and osculum.
| Aspect | Ostia | Osculum |
| Number in a sponge | Multiple ostia may be present on the sponge surface | Typically, there is one osculum, though large sponges may have multiple oscula |
| Location | Typically found on the body surface, often in the sponge walls | Positioned at the apex (top) of the sponge body |
| Function | · Inflow structures; allow water to enter the sponge · Water enters through ostia, brings in nutrients and oxygen | · Outflow structure; expels water from the sponge · Water that exits through the osculum, carries waste and excessive water |
| Size | Generally smaller in size | Larger in size compared to ostia |
| Structure | Small pores or openings in the sponge body | Larger opening, often surrounded by a collar or lip |
| Associated cells | Can be associated with pinacocytes (flattened cells) | Usually not directly associated with specific cells, but it’s surrounded by the choanoderm or a similar structure |