Not all nematodes are harmful to plants. The harmful ones Meloidogyne spp attack various parts of crops causing various damage depending on the species. Some nematodes inject toxic saliva to the roots as they feed resulting to abnormal cell growth-just like human cancer manifest-in the roots characterized by swelling called galls. The galls impair uptake of nutrients and water. Others result to root rot leading to total crop failure just like human cancer which is a terminal disease.
Cancer in basic definition involve abnormal body cells growing out of control. One of signs of cancer is indigestion or difficulty swallowing and this scenario, roots act like mouth of plants.