The ventral surface and sometimes more of the epidermis is ciliated. The waves of motion look much like those produced by the ciliary movement. More muscles yet are brought into play in looping. Then worm releases ant. Diagonal muscles etc.
Dorsoventral muscles permit flattening.
In all extracellular digestion is followed by phagocytic uptake of small particles by gastrodermis. Gastrovascular cavity has become a branched intestine in many. Some marine species have a larval stage called Mueller's larva. Reproduce asexaully by transverse fission Famous for regeneration.