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The anterior cavity receives the urogenital canal, and is the sole outlet of the urine and the sexual products; the hind or anus-cavity passes the excrements only. Even before this partition has been formed in the Marsupials and Placentals, we see the first trace of the external sexual organs.
The inner coat of the mouth-cavity is not provided by the gut-gland layer, but by the skin-sense layer; and its muscular substratum is provided, not by the gut-fibre, but the skin-fibre, layer. It is the same with the wall of the small anus-cavity.
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