Flatworms also have a feature that the sponges and cnidarians do not, a middle layer of actual organized cells. Flatworms have three cell layers: an ectoderm, an outer covering like the other groups have; an endoderm, a lining of the digestive spaces somewhat like a cnidarians; and a mesoderm between the other two. With actual tissues protected like this, another evolutionary explosion of features happened.
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Sponsored Topics. Portersville is a borough in Butler County, Pennsylvania, United States. The population was 268 at the 2000 census. Portersville is located at 40Â°55â²31â³N 80Â°8â²39â³W / 40.92528Â°N 80.14417Â°W / 40.92528; -80.14417 (40.925285, -80.144229).