ABSTRACT
Nematodes are unsegmented worms that typically are elongate and cylindrical in shape with tapered ends. Roundworms vary considerably in size from microscopic to over a meter in length. They possess a fluid-filled cavity or pseudocoel, and a complete digestive system comprised of an anterior mouth, a muscular esophagus (pharynx), an intestine, and a rectum that terminates posteriorly at the anus. The mouth may be surrounded by lips and a buccal cavity with cutting plates or teeth. Parasitic nematodes commonly feed on the semi-liquid contents of the hostOs alimentary canal, intestinal mucosa, blood or other body fluids, and various lysed tissues. In some forms, the muscular esophagus is cylindrical and virtually of a uniform diameter throughout, and the parasite is termed filariform. This type of esophagus generally characterizes infective stage larvae. In certain other nematodes, the esophagus is expanded posteriorly into a valved bulb, and the parasite is termed rhabditiform (Fig. 1). The latter type frequently characterizes the free-living larval stages. Some nematodes have both filariform and rhabditiform stages in their life cycles. The body is covered by a non-cellular cuticle, which may exhibit longitudinal ridges, striations, wart-like structures, lateral expansions anteriorly and posteriorly, and spines. Before reaching sexual maturity, all nematode larvae undergo a series of four molts or ecdyses. Cuticular structures of some importance are amphids and phasmids. Amphids are a pair of minute sensory organs, considered to be chemoreceptors that open on each side of the head. Phasmids are a pair of caudal (post-anal) organs similar in structure to amphids. Some phasmids are glandular and serve an excretory function, while others are sensory and believed to be involved in chemoreception.
