The body is built on the plan of "a tube within a tube."   The inner tube is the straight-line digestive tract, extending from the slit-like mouth on the first segment to the anal slit on the last segment. The outer tube, forming the body wall, consists of two layers of muscle, the moist skin, a cuticle, and a secretion of slimy mucus.  The fluid-filled cavity between the two tubes contains a well-developed circulatory system,  a nervous system,  numerous thread-like kidneys, and the reproductive organs.  In fact, the worm is exceptionally well organized,  more so than one would imagine from its lowly position in the animal world.