A glass sponge coordinates its whole body, and it has no nervous system

Hexactinellid sponges are a syncytium, one continuous cytoplasm. A single electrical impulse crosses the body and stops every feeding chamber at once. There are no neurons to carry it.

A glass sponge pumps water through chambers lined with flagella, and it lives on what the water brings. Sally Leys and George Mackie put electrodes on the tissue and stimulated it. One impulse travelled through the animal, and the pumping stopped, everywhere, together. The tissue that carried the impulse is not a nerve net. Glass sponges are built largely as a syncytium: the cells are fused, cytoplasm to cytoplasm, so an electrical change in one place is an electrical change next door without a synapse. Sediment in the water is the natural reason to stop. Pumping mud into the chambers would clog them. The stop is coordinated because the body is, electrically, one room.

Other sponges have no such impulse. They are cellular, and they plug individual pores slowly. The glass sponges found a different answer to being large: fuse the tissue, let a signal run, and accept that you will not have a brain, a synapse, or a private thought in any chamber. The arrest is the thought. It is one bit, and it is body-wide, and it is older in spirit than the nervous system it resembles.