Hatschek's pit

In the lancelet Hatschek's pit, not to be confused with Hatschek’s nephridium, is a deep ciliated fossa on the dorsal midline of the buccal cavity (the region of the gut behind the mouth) (1) Among other things, it secrets mucus which entraps food particles from the water.

References

(1) Jefferies, R.P.S., The Ancestry of the Vertebrates. Natural History Museum (London), 1986.