Phylum Nematoda (roundworms)

1. Parasitic.
2. Unsegmented.
3. Complex cuticle without cilia.
4. Intermal fertilization.
5. 1cm to 8m in length.
6. No circulatory system.
7. Alimentary tract present.

Nematodes are usually parasitic and have tooth or jaw-like structures that pierce prey and suck out the contents by a pumping action of the pharynx. Some free-living nematodes feed on bacteria. The exterior body covering is a noncellular, flexible, multilayered structure called the cuticle. The hypodermis is a layer of tissue beneath the cuticle that is responsible for the formation of the cuticle. Movement is controlled by longitudinal muscles. Nematodes maintain high internal pressures to keep them from collapsing and to aid in locomotion. Most nematodes are aerobic but can survive short periods of anaerobic conditions. A few forms can survive anoxia indefinitely. Most nematodes produce sperm or eggs, but some are hermaphroditic.

• Nematodes, best known for their damaging effects on crops, are also abundant in freshwater.
• The oldest fossil record of aquatic nematodes dates from the Upper Precambrian of Australia and Europe.
• Nematodes normally contribute between 1 and 15% of the zoobenthic biomass in lakes.

1. Barnes, R.S.K., P. Calow and P.J.W. Olive. 1993. The Invertebrates. Blackwell Scientific.
2. Thorp, J.H., and A. P. Covich. 1991. Ecology and Classification of North American Freshwater Invertebrates. Academic Press Inc.

Materials available for examination:

1. Preserved ascarid worms
2. Cross-sectional slides of male and female Ascaris
3. Miscellaneous slides of nematodes

Suggested Drawings:

1. Draw the whole worm and differentiate between male and females
2. Draw cross-sectional images of male and female worms. Draw them on the same page so the viewer can see the difference in the reproductive systems of males and females.
3. Make an enlarged drawing of the body wall. Show the cuticle, muscle fibers in mesodermal cells and the vacuolated portion of the mesodermal cells.

(Douglas J. Coish)