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This again is a sign that the muscles are becoming more important, for the excretory system is needed mainly to remove their waste. These tubes maybe only greatly enlarged glands of the skin. TURBELLARIAN. LANG. va and ha, front and rear branches of gastro-vascular cavity; ph, pharynx.
These seem to distribute the dissolved nutriment; hence the whole cavity is still often called a gastro-vascular cavity as serving both digestion and circulation. There is no anal opening, but indigestible material is still cast out through the mouth. The animal can gain sufficient oxygen to supply its muscles and nerves, which are the principal seats of combustion, through the external surface.
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