1.The main difference is that nephridia are found in invertebrates and nephrons are found in vertebrates. 2.One difference is that some invertebrates (annelids) have funnel-shaped nephridia, which are different from the tube-shaped nephrons in mammals. 3.Also, insect nephridia salvage water and expel only uric acid, which nephrons don't do as well (much more water ends up getting through to the bladder). 4.In flatworms, nephridia have ciliated flame cells as the end to maintain a current, and nephrons do not. 5.Also, nephridia are scattered through a worm's body and open directly to the exterior of the body, while nephrons are grouped together inside a kidney and connect to other internal structures.
Nephron is the unit of excretory system in    "vertebrates" whereas nephridia is the unit of excretion in  "invertebrates"