What is the difference between nematodes and other worms?

Answer 1

Nematodes are triploblastic (they have an ectoderm, mesoderm and endoderm) but also have a pseudocoelom (a closed fluid-containing cavity that acts as a hydrostatic skeleton to maintain body shape, circulate nutrients, and hold the major organs). They have a thick cuticle covering their body to protect them from digestion. Just inside their gut are the longitudinal muscles; unlike other worms (annelids), the nematodes lack a circular muscle and so their movement is restricted to a side-to-side whip. The female reproductive system consists of two highly folded ovaries and two oviducts that store eggs

Annelids (other worms) are also triploblastic. They are coelomate where the coelom is divided into segments and each segment can move independently. The most distinguishing feature of this group is segmentation, with the segments visible as rings around the body. Movement is by their circular muscles which expand and contract, moving the animal forward as well as side to side. Movement is also aided by the presence of bristles along the body.

So basically the main difference in the two phylum is the presence or lack of circular muscle which is an important evolutionary step in annelids as it allows more effective movement.