Separates muscles of body wall and digestive tract, so food moves down GI while you're walking Hydrostatic skeleton because fluid-filled spaces have pressure.
Muscles work against fluid.
In earthworms, snails, insects, reptiles, birds, mammals. Sperm surrounded by fluid Tough shell surrounds eggs. Secreted by female, shell keeps embryo from drying out. Hangs over, creating a cavity for gills. Radula - rasplike belt of teeth that is used to scrape off algae or eat plants.