STRUCTURE OF THE EARTHWORM 135 



leaves, grass, animal tissues of any kind, and the minute forms of 

 life found in the ordinary dirt. Quantities of this dirt are con- 

 tinually taken in by the animal and are passed through the 

 alimentary tract unaltered, to be defecated through the anus. 

 The apparatus for food digestion and absorption is much more 

 complicated than in hydra where digestion is largely intracellu- 

 lar. In the worm it is inter-cellular and occurs in cavities into 

 which the lining cells secrete digestive ferments. The anterior 

 end of the digestive tract is covered by a thick muscular wall. 

 This part, the pharynx (Fig. 54, ph) is used as a sucking mechanism 

 for drawing in food matters. Posterior to the pharynx is the 

 oesophagus, a thin-walled tube extending from about the 

 sixth or seventh somite to the eighteenth or nineteenth and 

 covered over from the seventh or eighth to the fifteenth by the 

 large yellowish vesicles of the reproductive system, and en- 

 circled by the fine "aortic arches" of the blood vascular system. 

 Posterior to the reproductive organs and on the ventral side of 

 the oesophagus are three pairs of bright yellow organs called the 

 calciferous glands the secretions of which serve to neutralize the 

 acids taken in with the food. Immediately behind the cal- 

 ciferous glands the alimentary tract expands into a larger thin- 

 walled pouch termed the crop which serves as a food reservoir. 

 The crop opens into a thick-walled reservoir or muscular pouch 

 called the gizzard where the food materials are ground up into 

 fine particles, the dirt, sand grains, etc., serving a useful pur- 

 pose in the process. Posterior to the gizzard from about the 

 twenty-sixth somite to the posterior end of the worm the diges- 

 tive tract consists of a uniform tube lined by secreting cells. 

 This tube is called the stomach-intestine from its combined func- 

 tions, and it is covered with a thick layer of brownish-yellow 

 glandular cells termed the chlorogogue cells. They are richly sup- 

 plied with blood vessels and are supposed to have some function 

 connected with excretion. Finally at the posterior end the 



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.) 



