24 Feeding and Defecation

Abstract

I. INTRODUCTION: NEMATODE HYDROSTATICS
The observation that the interior of a nematode is under pressure was first documented for Ascaris by Harris and Crofton (1957). The common observation that a healthy worm bursts when the cuticle is punctured shows that it is true for C. elegans as well. This pressure is the “hydrostatic skeleton” (Crofton 1966) that allows the worm to maintain its shape against forces (such as surface tension) which would tend to squash it flat and provides the tension against which body muscles act in locomotion. The internal pressure is crucial to understanding the design of the digestive system.

The central part of the digestive system is the intestine (Fig. 1, top), a flexible one-cell-thick epithelial tube that runs most of the length of the worm. It is composed of a series of toroids, the anteriormost consisting of four cells, and each of the rest of two cells (Fig. 1, bottom) (White 1988), with microvilli on the lumenal surface (Sulston et al. 1983; Albert and Riddle 1988). Little is known about the physiology of the intestinal cells. In addition to absorbing nutrients, they may secrete digestive enzymes into the lumen, are thought to store nutrients (White 1988), and are known to synthesize yolk proteins (Kimble and Sharrock 1983). The basal surface of the intestine forms most of the inner boundary of the pseudocoelomic space, which extends beyond the ends of the intestine to contact all tissues. This space is filled with fluid that can be seen to flow...