Vertebrates (deer, birds, fish - anything with bones!): characterized by an internal skeleton, a brain and a spinal chord - a concentrated nervous system.

Mollusks (bivalves, gastropods, cephalopods, etc.): separate neural masses.

Articulates (lobsters, spiders, insects, some worms - animals with joints & sections): 2 ventral nerve chords, transition between circulatory system of previous and tracheae.

Radiates (star fish, sea urchins & whatever doesn't fit above): radial symmetry, no circulation, no nervous system.