what are lichens? give to uses and examples of it.name the excretoryy organs of-(i)cockroach(ii)leech

Lichens are association between fungi and green algae or blue-green bacteria. It is believed that the alga contributes organic food from photosynthesis, while the fungus is protected from high light intensity and is able to absorb water and mineral salts. Lichens are used as source of food and also for medicinal purposes. Dirinaria picta, Gyalectales can be two examples. 

Excretory organs of leech are nephridia. The excretory organ of cockroach is the malpighain tubules