Distribution of electroreceptors (red) and mechanoreceptors
(blue) on the dorsum of the platypus bill.
Each receptor complex has a pit capped by a flower-petal arrangement that opens when the bill is immersed. The shape and size of electroreceptors is distinct from mechanoreceptors (smaller, with a dome in the centre of the pit that is the top of the push rod which relays mechanical stimuli to the base). The distribution of receptors can therefore be mapped with a high-power dissecting microscope.
There are 40,000 electroreceptors and 60,000 mechanoreceptors summed over all srurfaces of the bill.
The parasagittal stripe-like arrangement has a correlate in hte greater
sensitivity of tehe platypus to stimuli that originate from locations downward
and laterally. Such stimuli would produce field lines parallel to the stripes
on the bill, suggesting a connection between the bias of the system for
stimuli that produce parasagittal field lines and the parasagittal arrangement
of the stripes of electroreceptors.