This slide shows a stained cross section through the body of a common earthworm (Lumbricus terrestris).  On the outside of the worm is a thin, non-living cuticle that is secreted by the underlying hypodermis.  Beneath the hypodermis is a thin layer of circular muscles and a much thicker layer of longitudinal muscles.  Observe the large body cavity (coelom) lined by a thin layer of flattened cells that make up the peritoneum.  Much of the coelom is taken up by the intestine, which contains a conspicuous fold of tissue called the typhlosole.  It's thought that this structure serves to increase the surface area of the intestine for absorption.  Covering the outside of the intestine and most of the inside of the typhlosole is a specialized tissue made up of chloragogue cells.  These cells are involved in a variety of metabolic functions including the synthesis of urea, glycogen, fats and hemoglobin.