Membrane potentials

Difference between charge ions in and out of the cell--this difference produces a charge on the membrane
membrane can discharge decrementally or all or none
Cnidaria
nerve net
modalities
tactile
chemical
pressure
temperature
light
Platyhelminthes
nerve net-ectoderm system provides for reception of external stimuli
ladder type nervous system
nerve is simply a collection of nerve cell processes
nerve cells have  both processes and bodies
ganglion is simply a collection of nerve cell bodies
eye spots--visual pigments rhodopsin
"cerebral ganglia" brain

Nematodes
constant cell number, two cells function in nerve conduction
Annelids
neurotransmitters
acetylcholine