136 



ORGANS AND ORGAN SYSTEMS 



anchoring the animal in its burrow, while the anterior end moves 

 freely about in a small radius around the hole. 



C. INTERNAL STRUCTURE. A first impression is that the 

 internal structures of an earthworm 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 

 (Fig. 53). The outer tube, formed by the body wall, is strictly 

 speaking, not one continuous tube but a multitude of minute 



an 



c. 



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

 A and C, and transverse section B, showing the two tubes, the coelom, and the 

 dissepiments, a, Aortic loops; a/, digestive tract; an, anus; e.g., cerebral ganglia; 

 coe, coelom; c.v., parietal vessels; ds, dissepiments; d.v. , dorsal vessel; m, mouth; 

 n, nephridia; o, ovary; o.d., oviduct; s.i., ventral vessel. (From Sedgwick and 

 Wilson.) 



tubes (150 more or less), formed by the transverse partitions, 

 septa or dissepiments, attached to the body wall where the annuli 

 mark their positions. The cavities between these dissepiments 

 are termed coelomic cavities, or simply the coelom. The body 

 wall is relatively thick and muscular, being made up of epithel- 



FIG. 54. Anterior part of the body of the earthworm as it appears when the 

 dorsal wall is removed, ao, Aortic loops; ph, pharynx; e.g., cerebral ganglia; 

 oe, oesophagus; s.v., seminal vesicles; s.r., seminal receptacles; c.gl., calciferous 

 glands; c, crop; g, gizzard; d, dissepiment; s.i., stomach intestine; d.v., dorsal 

 vessel. (From Sedgwick and Wilson.) 



