Important because during the passage of water through the sponge, those same collar cells that created currents also capture food particles with their collars. However, it’s the much, much smaller system of collar cell-lined canals that run between the two that are most important right now. When you look at a sponge, the big oscula where the water exits are typically obvious, and in looking a little closer you can sometimes pick out the numerous, but much smaller, ostia where water goes in, too. What you should understand from this is that through the activity of numerous cells, water moves into, through, and out of a sponge via many canals, and that some of the canals are very small. Many sponges, like this Haliclona specimen, have more than one osculum, which can be easily seen. From there it passes out of a relatively large opening, called an osculum, back to the environment. This current moves water (and food particles carried in it) from the outside into the sponge’s body through numerous tiny openings, called ostia, then through lots of very tiny canals in the body, and then into a larger internal chamber called an atrium. These cells have a mesh-like collar at one end that encircles a tiny whip-like structure called a flagellum, and when thousands of them simultaneously wiggle these whips it creates a current of water. They’re also filter feeders that strain tiny food particles from the water, using specialized types of cells called choanocytes or collar cells, which line their interior surfaces. They stay attached to the bottom for their adult life, and with the exception of a few that can slowly move by re-arranging the placement of some cells, they stay in one place. Instead, they are collections of a few types of cells living together in an organized mass. Sponges are the simplest of the multi-cellular animals, lacking any sorts of tissues or organs, etc. ![]() Here you can see down into the osculum of a large sponge.
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