BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. DIVERSITY OF LIFE ON EARTH

2.8. Kingdom Animalia (animals)

2.8.4. Phylum Nematoda (nematodes or roundworms)

Nematodes are lower worms with a slender, cylindrical body tapering at both ends. Like Flatworms, they are triploblastic and lack a Circulatory system. They can be considered neither coelomate nor acoelomate animals (sect. 2.8.5); their internal body Structure is relatively simple and need not be discussed here in detail. Nematodes are nearly ubiquitous, inhabiting Water, soil (up to 100,000 million per hectare), and A wide variety of other environments. However, the majority are microscopic or too small to be easily noticed. New species are continually being discovered; one appears to be unique to the mash used in German beer brewing. Roundworms, trematodes, and tapeworms make up the three Major Groups of animal parasites. The Characteristic Features of roundworms are listed in Table 2.15. A typical example of a nematode is the roundworm (Ascaris; Fig. 2.52).

Class="center">Table 2.15. Characteristic Features of the phylum Nematoda (nematodes or roundworms)

Fig. 2.52. The roundworm (Ascaris lumbricoides), a widespread intestinal parasite of humans and pigs.

One species, Ascaris lumbricoides, is a common parasite of humans and pigs. It is a relatively large, cream-colored nematode, roughly the size of an earthworm (up to 20 cm in length). In severe cases, roundworm infection (ascariasis) can lead to intestinal obstruction. The eggs of Ascaris, passed out in the feces, are extremely resilient and remain viable for years. The male Ascaris is smaller than the female and has a more curved posterior end. Another well-known parasitic nematode, Wuchereriabancroftii, attacks The Lymphatic system, causing elephantiasis (Fig. 2.53). In this condition, the legs become massively enlarged, resembling those of an elephant. Nematodes can also infest plants, including a wide variety of cereal crops.

Fig. 2.53. A patient with elephantiasis.