Class Adenophorea includes Trichinella spiralis, the roundworm that can be passed on to humans by eating undercooked pork. Young roundworms can live in the muscles of pigs, as pictured above. Pork that is not cooked properly then is eaten by humans can lead to a Trichinella spiralis infection.

This is the Necator hookworm.

Class Secernentea includes many parasites including Dirofilaria, the heart worm that can infect dogs via a mosquito bite. Once infected, the young worms grow into thin filament threads that can eventually block the lungs and heart.

Class Secernentea also includes two important human parasites that cause worldwide health problems: Necator and Ascaris.

Necator is the hookworm known to infect humans and other animals, such as cats and dogs. A hookworm infection can occur when a person walks barefoot in soil that has fecal matter containing hookworms. The hookworms are able to bore into the skin through the feet and then travel to the small intestine where they feed off the nutrients of the host. The host's feces then contain eggs of the hookworms, thus repeating the cycle. Hookworm infection can cause a multitude of problems due to the loss of nutrients.

Ascaris is another important parasite of the class Secernentea that can infect animals, including humans. The cycle begins with the eggs in human feces. The eggs then live in the soil for years. If this infected soil is ingested, the eggs can grow into larvae inside the intestines. The larvae then enter the blood stream by forcing their way into the blood vessels of the digestive tract. This can lead to a multitude of problems, and can result in death. The worms can block the function of many vital organs, causing critical respiratory issues, or block the functions of the pancreas or gallbladder.