Nematoda (Roundworms)

Family: Nematoda
Roundworms are divided in two lower classifications, known as Adenophorea and Secernentea. Roundworms are divided into these two individual classes as the result of the absence or presence of the phasmid sensory organ as according to CK12. This phasmid sensory organ is an organ comprise of neurons or sensors that detect a presence and send this message to the brain of the nematode.

The main features of nematodes are that they are unsegmented, mostly parasitic, have the alimentary tract present, and contain no circulatory system.

Adenophorea
Most of these roundworms are not parasitic, but rather not depend on another organism for food. Free-living nematodes generally feed on bacteria, fungus, and protozoans. As you seen before, Adenophorea are absent of having a phasmid sensory organ, which is located at the end of a roundworm's tail. They look similar to Secernentea, have a thin structure and cilia to move through a variety of environments.

Secernentea
Most of these roundworms are parasitic, having suckers to collect nutrients from their host. They are primarily terrestrial, and can be distributed into an organism's body when the future host comes to contaminated dirt. This is why they are most common with cattle, as well as household pets like cats and dogs. Not only can they inhabit living animals, they can also suck energy from plants and invertebrates.