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Two osphradia present but no hypobranchial glands nor operculum.

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.

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.

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.

In the middle line of its ventral side we find the hypobranchial groove.