Digestive System.

http://en.wikipedia.org/wiki/Ascidiacea

The pharynx forms the first part of the digestive system. The endostyle produces a supply of mucus which is then passed into the rest of the pharynx by the beating of flagella along its margins. The mucus then flows in a sheet across the surface of the pharynx, trapping planktonic food particles as they pass through the stigmata, and is collected in the ridge on the dorsal surface. The ridge bears a groove along one side, which passes the collected food downwards and into the oesophageal opening at the base of pharynx.