Ventral Centralization & Cephalization

Planaria nervous system

flatworm

• ventral nerve cords connected by transverse nerves

• pair of anterior ganglia

Earthworm nervous system

• double ventral nerve chords

• segmental ganglia & nerves (contain sensory and motor neurons that branch off and innervate the body wall muscles)

• pair of anterior ganglia

Arthropods nervous system

arachnids, centipedes, crustaceans, insects

• double ventral nerve chords

• anterior ganglia form rudimentary brain (regional control, is decentralized and governed by multiple segmental ganglia)

Cephalopods

squid, octopus, cuttlefish, nautilus

• most advanced brain of the invertebrates

• two separate ventral nerve cords

Echinoderm (sea start)

central nerve ring with radial nerves that for an extensive epidermal nerve plexus (type of nerve net)