Phylum Nematoda: Roundworms

Nematodes have a more complex body plan than that of flatworms, but simpler than annelids. Free-living (non-parasitic) nematodes are extremely common in most kinds of habitats, but they are usually too small to see. Parasitic nematodes are also very common, and are often much larger.

Nematode features:

• Three tissue layers in embryo. Almost all animals share this basic feature; the sponges and cnidarians are exceptions.
• Pseudocoelomate: The inside of a nematode's body is mostly filled with empty space called the pseudocoel. This space might at first appear to be the same as the coelom in an earthworm, but it isn't. The pseudocoel develops in a different way (as will be covered later in the tissues and development lab), and, unlike a true coelom, is not lined with mesoderm-derived tissues.
• Complete digestive tract: Comparable to that of annelids, but nematodes show little or no specialization of different regions of the gut.
• No circulatory system: Nematodes don't have hearts, blood vessels, or blood. The fluid in the pseudocoel passively circulates nutrients and other substances throughout the body.
• Cuticle: a tough, flexible acellular (not containing cells) layer on the outside of the body. Because the cuticle isn't stretchy, nematodes must molt (shed the cuticle) in order to grow. Nematodes have this feature in common with arthropods such as insects and crabs.

Ascaris cross section (female)

Ascaris is an intestinal parasite in pigs. Parasitic nematodes tend to be much larger than free-living nematodes; that's one reason why they are common specimens for bio labs. (Another reason is that there are simply a lot of parasites out there.)

Digestive tract: Compare this slide to the whole preserved Ascaris specimens in lab. If you dissect one of those specimens, you'll see that the internal organs (gut and reproductive organs) are simply hanging free inside the body cavity, or pseudocoel. In this cross-section, the intestine doesn't appear to be attached at all, and if you look closely (at 40x magnification), you'll see that the wall of the intestine consists of just one layer of epithelium, with no other tissue layers attached.

If you look at the gut of an earthworm (phylum Annelida), you'll see that it's considerably more complex.

Reproductive organs:

• Ovary: makes the eggs.
• Uterus: stores eggs until they are released for external fertalization.

Muscles: Nematodes have only longitudinal muscles, running parallel to the length of the worm. The cuticle is flexible but resists shortening, so it acts as a flexible skeleton. The longitudinal muscles contract along one side of the worm at a time, allowing the body to flex back and forth. Unlike annelids, nematodes don't have circular muscles or a segmented body, so their movement is much more limited.

Ascaris cross section (male)

The male differs mainly in terms of its reproductive organs:

The testis is a long tube, looping back and forth throughout the body; in this cross-sectional view, there appear to be multiple separate testes. The testis makes sperm.

The vas deferens (also called ductus deferens) is another part of the same tubular male reproductive tract; it is responsible for storing and maturing sperm.

References & further reading

Ascaris suum on Invertebrate Anatomy Online by Richard Fox, Lander University

Caenorhabditis elegans on Wikipedia. One of the most-studied animals on earth.