Nematodes or roundworms are elongated, cylindrical and tapered at both ends. Adults of this class range from less than a millimetre in length to more than one hundred centimeters (Dioctophyma renale ). They have a complete digestive tract and a tough, elastic, skin-like cuticle. The mouth area may be specialised for attaching to or feeding on the host. Males of certain species of nematodes attach to females for mating by using a structure called a bursa. This is a posterior expansion of the cuticle or skin which is bell-shaped or funnel-shaped and supported by finger-like projections called rays.

Mating is also assisted by structures called spicules, used by the male to hold open the genital orifice of the female. The shape and arrangement of the male bursa and spicules vary from species to species and are frequently used to identity different nematodes.

Internal organs showing predilection sites *

* The following parasites are not prevalent in New Zealand: Ancylostoma, Dictophyma renale, Filaroides, Trichuris, Physaloptera, Spirocerca and Dirofilaria.

Life cycle of a typical roundworm

Life cycle of a hydatid tapeworm (Officially eradicated in New Zealand in 2002)

Tapeworms, or cestodes, are flat, ribbon-like organisms that live most often in the small intestine of their host. The head, or scolex, of the tapeworm has suckers hooks, or a combination of suckers and hooks used to attach the worm to the wall of the intestine. Proglottids (tapeworm segments) are generated from the scolex. In some species, the strobila or body of the worm may become several meters long. Each mature proglottid is a complete functional unit, incorporating a digestive system, organs of both sexes and other organs. This phenomenon of both sexes in one body is known as hermaphroditism. Cestodes absorb nourishment directly through their skin from the gut contents of the host animal.