STRUCTURE OF THE EARTHWORM 



133 



the animal in its burrow while the anterior end moves freely 

 about in the small radius around the hole. 



C. INTERNAL STRUCTURE. Fig. 53 is a diagram of a worm 

 cut from end to end in a vertical dorso-ventral plane. A first 

 impression is that the internal structures consist of a tube within 

 a tube, the inner tube being the alimentary tract continuous 

 from the mouth at the anterior end to the anus at the posterior. 

 The outer tube, formed by the body wall, is, strictly speaking, 

 not one continuous tube but^a multitude of minute tubes (150 



FIG. 53. General diagram of the earthworm as seen in longitudinal section 

 showing the two tubes, the coelom, and the dissepiments. For identification of 

 parts see stereogram, Figs. 55 and 57. (From Sedgwick and Wilson.) 



or more), formed by the transverse partitions, septa or dis- 

 sepiments attached to the body wall where the annuli mark 

 their positions. The cavities betwee'n these dissepiments are 

 termed coelomic cavities or simply the coelom. The body wall is 

 relatively thick and muscular, being made up of epithelium, 

 muscles, nerves, glands, connective tissue, blood vessels, and 

 endothelium, and the whole is covered on the outside by a deli- 

 cate lifeless coat termed the circle. 



The Digestive System. The food of an earthworm consists of 



