They are generally free-living or parasites. They are found in the soil, freshwater or as a parasite on the plants, animals and even the humans. Ascaris is also found living as an endoparasite in the human intestine.
The body is bilaterally symmetrical and also triploblastic.
The body maybe unsegmented, long, and tapers at the end. Metameric segmentation is generally not found.
They are dioecious, males are usually smaller than the females.
Animals of this phylum also possess pseudocoelom (false coelom).
They have an organ-system level of the body organization.
The digestive system usually comprises of complete alimentary canal with the muscular pharynx. In Ascaris, the rectum opens in the anus in females and cloaca in the males, present posteriorly.