What specific structures are in electroreceptors?

Electroreception is limited to aquatic environments because on here is the resistivity of the medium is low enough for electric currents to be generated as the result of electric fields of biological origin.  In air, the resistivity of the environment is so high that electric fields from biological sources do not generate a significant electric current.  Electroreceptor are found in a number of species of fish,  and in at least one species of mammal,  the Duck-Billed platypus.

You need to be more specific.  i really don't know what a "motion map" is.you can draw a graph of an object's position, distance, displacement, speed,velocity, or acceleration versus time, or you can draw a graph of the object's x-position versus its y-position and not give any information about time at all.if the graph presents position or displacement against time,  then constant velocity is a straight line . . i.e. a line that has  constant slope.if the graph shows speed against time, then constant velocityis a horizontal piece of line.if the graph shows acceleration against time, then constant  velocity is a piece of line that lies on the x-axis . . it's value is zero.