The Nematode Body Plan

The phrase tube-within-a-tube is a convenient way to think of nematode body structure, and also a term used to refer to a major trend in the evolution of triploblastic metazoa (Brusca and Brusca, 1990 Invertebrates). It refers to the development of a fluid-filled cavity between the outer body wall and the digestive tube. The nature of this body cavity has led to the grouping of metazoa into three grades, acoelomate, pseudocoelomate, and eucoelomate (figure from Brusca and Brusca,1990) . Nematodes together with Rotifera, Gastrotricha, Kinorhyncha, Nematomorpha, Acanthocephala, and Entoprocta are traditionally grouped together as pseudocoelomates, on the basis of possessing a body cavity that is not formed from the mesoderm or fully lined by peritoneum. However, there are some problems in applying this concept to nematodes. Cell lineage studies on the nematode Caenorhabditis elegans have demonstrated that most tissues in the nematode are of mixed lineage, derived from several different sources of embryonic tissue (see Bird and Bird, 1991 The Structure of Nematodes). Also, not all nematodes retain a spacious fluid-filled cavity, as can be seen in a cross section of the esophageal region of the monohysterid marine nematode, Theristis.