PHYLUM NEMATODA - ROUNDWORMS
Although they are less well known than annelid worms, the nematodes (sometimes called roundworms) are very diverse and of great ecological and economic importance. They are unsegmented worms that are bilateral and triploblastic, with a complete digestive tract and a pseudocoelom. A pseudocoelom is a body cavity that develops between the mesoderm externally and the endoderm of the gut tube internally. The development of a body cavity (coelom) is considered a major evolutionary advantage over those animals which do not possess a body cavity (acoelomate). Body cavities are advantageous for a number of reasons, such as to provide more room for organ development, to provide an increased surface area for diffusion of gases and/or nutrients, and to facilitate locomotion by serving as hydrostatic skeletons.
You will examine a member of the phylum Nematoda (Ascaris) as a representative pseudocoelomate. An intestinal parasite as an adult, there are species of Ascaris that occur in pigs, horses, and man.
1. External anatomy
Take an Ascaris from the supply, and place it in a dissecting pan. Gloves should be worn, if available. If not, handle organism with forceps. Do not touch it with bare skin. You will need to obtain a female specimen for the dissection below, however you should examine a male specimen externally as well. The sexes are easily differentiated. Male worms are smaller, typically have a hook-shaped sideways bend near their posterior end, and may have tiny copulatory spicules protruding slightly from the cloaca. You should also find and observe the triradiate mouth at the anterior (smaller) end.
2. Internal anatomy
Your instructor may provide a prosected specimen of Ascaris for you to observe or may instruct you to make your own dissection following the directions below.
Lay the worm down in the dissecting pan, and place a pin through the anterior and posterior ends. Use a dissecting needle or fine scissors to cut it open full length along the dorsal midline. Pin back the body walls to expose the internal organs. You may either work dry or, if you prefer, flood the pan (and specimen) with about a quarter inch of water.
Draw the dissected specimen. Locate, and label, the following structures:
- pseudocoel (body cavity)
- intestine (nonmuscular and formed only by endoderm)
- vagina and uteri - the "Y" shaped structure (the short, stem portion of the "Y" is the vagina; the arms of the "Y" are uteri)
- ovaries and oviducts - the mass of coiled tubules connected to the uteri (the tubules coiled around the arms of the "Y" are the oviducts; the smaller, terminal portions are the ovaries)
Figure 1. Anatomy of the roundworm Ascaris. (from http://kentsimmons.uwinnipeg.ca/16cm05/16labman05/lb5pg9.htm)
Obtain a slide of a cross-section through a female Ascaris. Observe this cross-section using a compound microscope. (A diagram of a cross section through a female Ascaris can be viewed here.) Draw the specimen, labeling the pseudocoel, intestine, uteri, oviducts, muscle cells, dorsal nerve cord, and ventral nerve cord. In some sections, you may also be able to see the excretory canals. You may refer to the diagram below to aid in interpreting your slide, you must make your sketch from the slide. Remember to label your drawing with the total magnification under which the slide was observed. Can you identify two important differences between this slide and the cross-section of Lumbricus that you viewed in the lab on annelid worms?
Figure 2. Cross-section of a nematode (from Brusca & Brusca, 2003, Invertebrate Zoology, 2nd ed.).
Here is a video of an Ascaris dissection.
4. Observation of Vinegar Eels (Turbatrix)
You should make a wet-mount slide of vinegar eels (Turbatrix aceti), a non-parasitic, free-living nematode that is found in unpasteurized vinegar. Observe the animals under low power using a compound microscope. Note the cylindrical body and tapering tail that is typical of nematodes. Also, note the characteristic whip-like movements that the animals make. Nematodes have longitudinal muscles, but lack the circular muscles found in annelid worms. Their thrashing movements are produced when the longitudinal muscles pull against the rigid cuticle and an internal hydrostatic skeleton formed by fluid in the pseudocoelom.