Nematodes (roundworms)


Nematodes are spindle-shaped worms, usually cylindrical and pointed at both ends, and without appendages. The worms are unsegmented and the cuticle or integument may be smooth or adorned with annulations, longitudinal striations or cuticular bosses.

The mouth is situated at the anterior end of the worm – terminally, subdorsally or subventrally. It is surrounded by lips; 3 in the more primitive forms, and 6 in the higher forms. In some cases, lips may be completely absent and then additional structures such as leaf crowns may be present (Strongylidae) or absent (Filariidae). The mouth leads to the buccal capsule, which may be well developed in some worms, or very small and weakly developed in others. Teeth may or may not be present in the buccal capsule. The oesophagus is a muscular tube with a triradiate lumen. The wall is thick and muscular and may contain glands that secrete digestive enzymes. There are marked variations in the shape of the oesophagus – the strongyliform and the rhabditiform types being the commonest seen in parasitic nematodes. The intestine is a simple, straight tube; in the female it ends in the anus and in the male opens into the cloaca (Figure 121).

Nematodes lack both circulatory and respiratory systems and oxygen is absorbed from the surroundings or produced by the worm’s own metabolic processes.

The ‘brain’ is the nerve ring around the posterior 1/3 of the oesophagus. A few ganglia are present and these may send ramifications to different parts of the body. The ‘nerves’ usually run along the dorsal and ventral lines of the worm.
Tactoreceptors occur as cervical papillae or pre-bursal papillae in males, and as pre and post-cloacal papillae in the females near or posterior to the vulva.

Figure 121 The anatomical structures of a nematode (redrawn from Chitwood & Chitwood, 1950).

The male reproductive organs consist of a testis, a vas deferens that is dilated for part of its length, and a muscular ejaculatory duct. In most species there are two spicules, which are situated dorsally immediately in front of the cloaca. They serve as organs of attachment and to dilate the vulva of the female during copulation, directing the sperm into the vagina. Frequently there is a small sclerotised organ dorsal to the spicules. This is the gubernaculum, which guides the spicules. A thickening on the ventral wall is known as a telamon. In some species both are present, in other species only the gubernaculum, and in still other species both are absent. The males of many of the worms possess a copulatory bursa supported by rays (Figure 122). In other species there are cuticular thickenings known as caudal alae. These have the same function as the copulatory bursa. They are not supported by rays but may be supported by pedunculated genital papillae (Figure 123).

Figure 122 Bursal structures of strongyloid nematode male.

Figure 123 Bursal structures of an ascarid or spiruroid male.

Figure 124 Alternative naming of the bursal rays.

Figure 125 Male reproductive system, lateral view.

The female reproductive organs (Figure 126) consist of 2 ovaries leading to oviducts, dilated seminal receptacles, uteri, an ovejector, a vagina, and a vulva. The position of the vulva in relation to the body length is an important morphological character distinguishing families and genera and will be discussed under the different families. Uteri that run in opposite directions are known as amphidelph uteri, those that run parallel from the front to the rear as opisthodelph, and those that run from the rear to the front as prodelph. The morphology of the ovejector is also a diagnostic tool and it may be opposed, Y-shaped, or a single muscular organ. The different types will be discussed under the families. Females can be oviparous (egg-laying), ovoviviparous (eggs that contain a larva that will hatch immediately upon being laid), or viviparous (live-bearing).

Figure 126 Generalised representation of the female reproductive system.


Subclass Adenophorea

Few or no male caudal papillae. Phasmids (a pair of specialised sensory organs on the tail) absent. Oesophagus cylindrical or with glands free in the pseudocoelome forming a stichosome or trophosome. Eggs with polar plugs. Eggs may hatch in utero; L1 often have a stylet and are infective to the host.

Order Enoplida

Superfamily Trichinelloidea

The common morphological feature of ‘Trichinelloids’ is the stichosome oesophagus, which comprises a capillary-like tube surrounded by a single column of cells. The male has one or no spicule. Female reproductive system monodelphic (= single reproductive system). L1 usually infective in egg or free. In small or large intestine of mammals and birds.

Family Trichuridae

Male with single spicule enclosed in a spicular sheath sometimes referred to as a cirrus. Spicular sheath may be armed with spines. Vulva opens near posterior end of oesophagus. Eggs oval with polar plugs.

Genus Trichuris

Body with slender anterior region and thicker, shorter posterior region. Called ‘whipworms’ (trichos = hair). Single bacillary band present, lateral to oesophagus, terminating near its posterior end. Stichosome with single row of 40-200 stichocytes. Male cloaca terminal or subterminal. Female vulva opens near junction of narrower and wider parts of body. Thick muscular vagina...

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