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Under this is a soft medullary substance, which consists of entodermic cells with vacuoles. These gonads are not yet independent sexual glands, but sexually differentiated cell-groups in the medullary substance, or, in other words, parts of the gut-wall. Most of the Platodaria have not the muscular pharynx, which is very advanced in the Turbellaria and Trematoda.
On these principles I assume that the oldest and simplest Turbellaria arose from Platodaria, and these directly from bilateral Gastraeads. The chief advances were the formation of gonads and nephridia, and of the rudimentary brain.
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