Lesson 4: Phylum Nematoda: Roundworms
Roundworms are hardly ever seen, but they are very numerous. Many species are parasitic and most groups of marine organisms have nematode parasites. They are cylindrical with pointed ends. They have a complete digestive system and a body cavity (pseudocoelom) that is filled with fluid that transports nutrients. Nematodes have a hydrostatic skeleton. This means that the layer of muscles pushes and squeezes against the fluid in the body cavity to provide support and aid in the locomotion of the organism.
One nematode, Anisakis, inhabit the intestines of seals and dolphins. Their larvae, however, are found in the flesh of many fish and can infect humans who consume raw or poorly cooked fish. If they are not vomited or coughed up, the larvae penetrate the wall of the stomach or intestines, causing symptoms similar to ulcers. If you eat sushi or cechive, this is a risk that you must take.
Photo: Colorized electron micrograph of soybean cyst nematode (Heterodera sp.) and egg; Wikipedia.org
View this video illustrating the hydrostatic skeleton and whip-like movement of nematodes.
Video Attribution
Description: Nematode Movement
Source: Uploaded by Scubaman5000 on YouTube