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In some there is another gland besides the liver, and this is taken to represent the kidneys. The body-cavity proper, or coeloma, which is filled with blood and encloses the hepatic gut, is very narrow in the Ascidia, as in the Amphioxus, and is here also usually confounded with the wide atrium, or peribranchial cavity, full of water.
In the middle line of its ventral side we find the hypobranchial groove. The ejection-opening of this peribranchial cavity corresponds to the branchial pore of the Amphioxus. In the adult Ascidia the branchial gut and the heart on its ventral side are almost the only organs that recall the original affinity with the vertebrates.
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