What are nephridia? Explain the major types of nephridia in detail.

Nephridia are simple or branching tubules used for excretion which open to the exterior through pores called nephridiopores. 

Two major types of nephridia are as follows: 

i. Protonephridia: 

These are network of dead end tubes called flame cells. They are mostly found in animals that lack a true body cavity, e.g. Platyhelminthes, rotifers, some annelids and Amphioxus. 

ii. Metanephridia: 

These are unbranched coiled tubes that are connected to the body cavity through funnel like structures called nephrostomes. Body fluid enters the nephridium through nephrostome and gets discharged ‘ through nephridiopore. e.g. Earthworms.