The epipharyngeal groove is a ciliated groove along the dorsal side of the inside of the pharynx in some plankton-feeding early chordates, such as Amphioxus. It helps to carry a stream of mucus with plankton stuck in it, through the pharynx into the gut to be digested.
The subnotochordal rod or hypochord is a transient structure that appears ventral to the notochord in the heads of embryos of some vertebrates. Its appearance is stimulated by a chemical secreted by the notochord. The subnotochordal rod helps to stimulate development of the dorsal aorta.
There is an opinion that these two structures are homologous.
References
Epipharyngeal groove Wikipedia(Text) CC BY-SA