Most vertebrates have digestive system incomplete tube within a tube sac-oike none of the above

Most vertebrates have a tube within a tube.  The skin forms a tube that is open at the mouth and the anus.  The mouth and the anus are the ends of another tube found inside the tube of skin.

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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