PARASITOLOGY

n 3.11 NEMATODES

General characteristics:

n All nematodes have a non-segmented cylindrical body. They are triploblastic with a

fluid-filled body cavity known as a pseudocoelome (not a true ceolome).

n They have both free-living and parasitic forms. The free-living forms inhabit fresh-

water, marine and terrestrial environments. Most free-living forms are relatively

small (only just visible to the naked eye).

n Adult parasitic forms are found in both plants and animals. The adult parasitic forms

range in size from microscopic to 10 cm plus.

n 3.11.1 MORPHOLOGY

A typical nematode has an elongated cylindrical shape, limbless without cilia and no

respiratory organs (see Fig. 3.8). A typical nematode has the following body openings:

n A mouth at the anterior end. Slightly behind are small openings for the excretory

organs — the amphids.

n The female opening — a genital pore — midway along the body length.

n Near the posterior end is the anus, the rear opening of the alimentary canal.

n In the male the genital organ opens almost at same place as the anus.

n 3.11.2 INTERNAL ANATOMY

The internal structure of a nematode consists of two ‘tubes’, the alimentary canal and

the reproductive organs, contained within a fluid-filled cavity, the pseudoceolome (see

Fig. 3.9). The outer cuticle is a non-permeable, multi-layered structure. On the inner

side of the cuticle is the hypodermis consisting of a layer of longitudinal muscles. In

addition there are unique types of muscles cells which ‘suspend’ the gut and repro-

ductive organs.

The mouth is located at the tip of the anterior end and opens into a buccal cavity. This

leads into a ‘muscular’ pharynx which in some species is subdivided into a series of

‘bulbs’. The internal shape tends to be triradiate. The structure of the mouth parts and

the pharynx are adapted to the feeding habits of the different species. The pharynx opens

into a simple tube-like intestine with a subterminal opening, the anus.

The nervous system consists of a series of nerve commisures that are located in the

anterior ‘head’ region and surround the pharynx. A series of large ganglia form a ‘brain’.

A dorsal and ventral nerve chord and two lateral nerve chords lead out from the main

anterior ganglia. These nerve chords run through the hypodermis of the cuticle.

n 3.11.3 REPRODUCTIVE SYSTEM

In most cases the sexes are separate. In those species that are hermaphrodite the male

gonads mature first (protandry). There are also nematode species that are parthenogenic.

PARASITOLOGY

Mouth

Buccal cavity

Oesophagus Pseudocoel Anus

• Figure 3.8A nematode Intestine
  is a round cylindrical worm
  with a fluid-filled body
  cavity, the pseudocoel.
  At the anterior end is an
  opening, the mouth, that
  leads into the buccal
  cavity, the pharynx and
  then into the intestine
  which runs the length of
  the body. The gut ends in
  an opening, the anus, at
  the posterior end.