The Roundworms (Nematoda)

Abstract

This group of animals contains a large number of ‘worms’ with long cylindrical bodies, spindle shaped or sometimes thread-like. The body is usually pointed at both ends and it shows no sign of segmentation such as is characteristic of the true worms. On the outside is a clear elastic cuticle which is shed four times during the period of development from egg to adult. Near the anterior end is a mouth (which is lacking in some forms). This leads through an enlarged fore-gut often with a bulbous portion (Fig. 14) to the main part of the alimentary canal, which is a straight tube leading to the anus at the posterior end of the body. The mouth is sometimes armed with hooks and stylets. Individuals are of separate sexes developing either sperms or eggs in a large mass of cells alongside the gut. The female has a special exit for the eggs, but in the males the sperms are passed out through the anus. A curious feature of the Nematodes is that the tissues of which their bodies are composed are made up of very few large cells; there are no cilia anywhere, no blood system, and no breathing organs. The eggs are covered by a hard shell. The young Nematode hatches looking very much like an adult except for size. Sometimes they hatch inside the body of the female, but they generally do so after the eggs are laid.