Phylum nematoda (nematodes or roundworms)

  • They are triploblastic
  • Bilaterally symmetrical
  • Possess elongated, round ‘worms, with pointed ends
  • Unsegmented (like flatworms, but unlike annelid worms)
  • Alimentary canal with mouth and anus
  • Sexes are separate
  • Some free living, many are important plant and animal parasites
  • Anterior end shows a degree of cephalisation (development of a head)

The nematodes are simple worms, with slender cylindrical bodies tapering at each end. Like platyhelminths they are triplobastic with no blood system.

They are neither acoelomate nor coelomate, but have a simple internal body structure which we need not consider here. Although extremely common in water, soil (as many as 100 billion per hectare), and a wide range of other habitats, these worms are mostly microscopic or too small to be seen easily.

New species are constantly being discovered; one appears to be unique to German beer mats. The roundworms, flukes and tapeworms form the three largest groups of animal parasites.

An example is Ascaris. One species, Ascaris lumbricoides, is a common parasite of the intestine of humans and pigs. It is a creamy white colour and relatively large, about the size of an earthworm (up to 20cm long).

Heavy infections can cause obstructions in the gut. The eggs, which pass out in the faeces, are very resistant and can survive for years.

The male is smaller than the female and is more curved at its posterior end.

Another well-known nematode parasite is Wuchereria broncroftii, which infects the human lymphatic system and causes elephantiasis. The legs can swell enormously, resembling those of an elephant. Nematodes can also attack plants, including a wide variety of crops.

summary of the characteristics of phylum nematoda

  • The body is minute to quite lager
  • Elongated and cylindrical
  • Coelomate/pseudocoelomate
  • The anterior end is usually pointed
  • Movement is muscular with characteristics whipping flexure which is due to the animal possessing only longitudinal muscle fibres
    -The epidermis is syncytical and covered with an eclectic cuticle
  • The gut opens posteriorly through the anus which is sub-terminal
  • The nervous system is a network of cell which are aggregated at the anterior end of the body to  form a circum-oesophageal ring
  • Typically, the excretory system comprises two laterally placed intre-cellular tubes
  • Flame cells are absent
  • The sexes are usually separated
  • No cilia
  • Body not segmented
    -The gonads and their duct from continuous  tubes
    -The larvae are very similar to the adult
  • The life history is often complex and many forms are parasitic either in adult or in the larvae
  • Stages or throughout the entire life cyclic
  • Bilaterally symmetrical
  • Triploblastic body layer