Open circulatory system has low blood pressure, tissues aren't efficiently oxygenated, animals are slow moving. Waste from coelom into funnel, out other end in excretory pore. Flatworms have protonephridia are blind tubes that only open to outside. Flat bodies with 8 separate plates on shell. Torsion - twisting of visceral mass.
Larva is bilateral, then one side grows faster than the other.
Visceral mass twists 180 degrees, with U- shaped digestive system. Anus above head. May let head enter shell first, rather than foot. Elastic ligaments open shell, adductor muscles close the shell. Shell is calcium carbonate, secreted by mantle.