Nematodes & cestodes

sushi image by Maryna Girard from

Both cestodes and nematodes belong to the helminth, or worm, class of organisms. These multicellular eukaryotic organisms act parasitically upon host organisms, meaning they live inside another creature. But cestodes and nematodes belong to distinct phylum groups, or divisions, within the animal kingdom.

Cestodes biologically classify in the platyhelminths phylum, while nematodes classify in the nemathelminths phylum.

Characteristics of Cestodes

Cestodes exhibit a flattened body that can reach up to 6 meters in length. The worms are segmented and hermaphroditic--each segment contains both male and female reproductive organs. Cestodes do not have a digestive canal. Instead, they feed on predigested nutrients from their host with their scolex, or sucking head.

Characteristics of Nematodes

The term "nematode" derives "nema" from the Greek for "thread." Indeed, this is what nematodes resemble--a thread from 1 millimetre to 1 meter in length. Nematodes reproduce sexually and possess digestive and circulatory systems. Males tend to be shorter in length than females.


Cestodes, also known as tapeworms, act only endoparasitically: they live inside an animal or insect host. Cestodes affect much larger organisms such as birds, dogs and humans. To access these animals, cestodes use an intermediary animal or insect, such as beetles, snails, cows and flies, to reach the larger host. Nematodes, also known as roundworms, live inside other animals as well as externally. The external, free-living nematodes prey on organisms such as soil bacteria.

Nematode Life Cycle

The life cycle of the parasitic nematode begins when a host organism ingests a fertile and developed egg or eggs into its digestive tract. The egg lodges itself in the host's small intestine, where it hatches. The body's portal artery carries the hatched larvae to the lungs. The larvae develop further in the lungs and migrate to the throat. The host swallows the larvae back into the small intestine, where the larvae develop into adult worms and live up to 18 months inside the host, reproducing up to 25 million eggs.

Cestode Life Cycle

Cestodes also begin their life cycle when an egg, or larval cyst, is ingested by a host organism. However, the cestode's life cycle differs from that of the nematode in several important ways. First, the cestode larvae remain inside the host's small intestine, grabbing on to the intestine's mucous lining with their scolex. The worm grows and develops segments, which are excreted in the host's faeces. An intermediate host ingests the segments, which then create new larval cysts. Also unlike parasitic nematodes, cestodes can live up to a whopping 25 years inside a host.