In which animal nerve cell is present but brain is absent : -

Hydra (a cnidarian) has nerve cells arranged as a diffuse nerve net throughout the body wall, but no brain or central nervous system.
Sponges have no nerve cells at all.
Earthworm and cockroach have a brain (cerebral ganglia) and a ventral nerve cord.
So in Hydra nerve cells are present but the brain is absent.