PHYLUM NEMATODA

            Members of  Phylum Nematoda are worms with long, cylindrical bodies and for this reason are commonly referred to as roundworms.  The phylum Nematoda is among the largest of the animal phyla.  Although only approximately 15, 000 species have been named; biologists estimate that if all the species were scientifically described, the number would be closer to 500,000.  Most nematodes are found free-living within the soil, but a great many are important parasites of plants and animals.

 

Free living nematode.

Another soil nematode.

            Roundworms have two morphological advances absent in the flatworms.  First, in addition to their digestive cavity, roundworms have a body cavity called a pseudocoelom consisting of a fluid-filled space between the body wall and digestive tract.  Term - pseudocoelom is  explained in more detail at the end of this laboratory, in section IV.  Second, nematodes have a complete digestive tract with a mouth and an anus.

In this laboratory, you will examine Ascaris lumbricoides, which is a large nematode that infects the intestinal tract of humans and other vertebrates. It is one of the best-known parasitic nematodes. 

            Use forceps to obtain one Ascaris for your group and place it in a dissecting pan with a small amount of water.  You will also need several dissecting pins.  Compare the external characteristics of females and males. Males are smaller than females and have a hooked posterior end.  At the anterior end, locate the mouth surrounded by three lobes (lips) of tissue (you will need to use dissecting microscope for this exercise). At the posterior end, locate the anus (Figure 11).  The three lobes (lips) will help you distinguish the anterior from the posterior end of the animal. 

Why is the tripartite lip arrangement advantageous for Ascaris?

 

Figure 11:  Ascaris sp – external view of female and male.

Feeding and Digestion in Nematoda

            The specific feeding method utilized by a particular species of nematode is a reflection of its habitat and way of life.  Therefore, nematodes exhibit a wide range of feeding habits: carnivorous, herbivorous and parasitic.  Although the diet may be extremely varied, the nematode digestive system is relatively uniform throughout the phylum.  You will examine the parts of the digestive tract during your dissection of a roundworm.  The mouth opens into a muscular pharynx, which acts as a pump to bring food from the mouth into the intestine (You can examine the digestive system later once you dissect your  specimen). From the pharynx, a long tubular intestine travels the length of the body to the anus.  Digestive enzymes are produced by the single layer of epithelial cells lining the intestine.  Digestion begins extracellularly within the intestinal lumen but is then completed intracellularly.

Respiration and Circulation in Nematoda

            Nematodes lack respiratory and circulatory systems.  The majority of free-living nematodes are less than 2.5mm in length and are often microscopic.  These animals are largely the inhabitants of the interstitial spaces of aquatic sediments and soils.  In the soil, free-living nematodes live in the film of water surrounding each soil particle, so they are essentially always in a moist environment.  Free-living species obtain their oxygen requirements by diffusion from the environment.