138 ORGANS AND ORGAN SYSTEMS 



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

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

 delicate lifeless coat termed the cuticle. 



The Digestive System- -The food of an earthworm consists of 

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

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

 continually taken in by the animal and are passed, unaltered, 

 through the alimentary tract 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 mech- 

 anism 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 fourteenth or fifteenth, covered 

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

 yellowish vesicles of the reproductive system, and encircled 

 by the fine "aortic arches" of the blood vascular system. Pos- 

 terior 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 calci- 

 ferous 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. These are richly 

 supplied with blood vessels, and are supposed to have some func- 

 tion connected with excretion. Finally, at the posterior end, 



