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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.
As Wilhelm Muller, of Jena, has shown, this rudimentary organ is the last relic of the hypobranchial groove, which we considered in a previous chapter, and which runs in the middle line of the gill-crate in the Ascidia and Amphioxus, and conveys food to the stomach.
The median congenital fissures of the neck are probably caused by defective union of the branchial arches, although Arndt thinks that he sees in these median fistulas a persistence of the hypobranchial furrow which exists normally in the amphioxus. They are less frequent than the preceding variety. The most typical form of malformation of the esophagus is imperforation or obliteration.
Along the ventral side of the branchial sac runs a ciliated groove the hypobranchial groove which we have previously found at the same spot in the Amphioxus. The food of the Ascidia also consists of tiny organisms, infusoria, diatoms, parts of decomposed marine plants and animals; etc.
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