250 PHYLUM ANNELIDA 



Alimentary system. — Earthworms eat the soil for the 

 sake of the plant debris which it may contain, and also 

 because one of the modes of burrowing involves swallowing 

 the earth. In eating they are greatly helped by the muscular 

 nature of the pharynx ; from it the soil passes down the 

 gullet or oesophagus, first into a swollen crop, then into a 

 strong-walled grinding gizzard, and finally through a long 

 digestive and absorptive stomach-intestine. There are 

 three pairs of oesophageal glands. Canals from the posterior 

 two pairs open into the anterior pair, and thus into the 

 gullet. Their contents are Hmy, and perhaps counteract 

 the acidity of the decaying vegetable matter. It may be 

 that they are in part excretory ; or it may be that they 

 serve to fix some of the carbon dioxide formed by the 

 animal. The long intestine has its internal surface 

 increased by a dorsal fold, which projects inwards along 

 the whole length. In this " typhlosole," and over the outer 

 surface of the gut, there are crowded yellow cells. 



There is no warrant for calling the yellow cells hepatic or digestive. 

 Structurally they are pigmented cells of the peritoneal epithelium, which 

 here, as in most other animals, lines the body cavity and covers the 

 gut. As to their function, they absorb particles from the intestine, 

 and go free into the body cavity, whence, as they break up, their 

 debris may pass out by the excretory tubes. When a worm has been 

 made to eat powdered carmine, the passage of these useless particles 

 from gut to yellow cells, from yellow cells to body cavity, and thence 

 out by the excretory tubes, can be traced. The amoeboid cells of the 

 body cavity fluid act as phagocytes. Various ferments have been 

 detected in the gut, a diastatic ferment turning the starchy food into 

 sugars, and others — peptic and tryptic — not less important. The wall 

 of the stomach-intestine from without inwards, as may be traced in 

 sections, is made up of pigmented peritoneum, muscles, capillaries, 

 and an internal ciliated epithehum. In the other parts of the gut the 

 innermost lining is not ciliated, but covered with a cuticle. 



Vascular system. — The fluid of the blood is coloured 

 red with haemoglobin, and contains small corpuscles. 

 Along the median dorsal line of the gut a prominent blood 

 vessel extends, another (supra-neural) runs along the upper 

 surface of the nerve-cord, another (infra-neural) along the 

 under surface, while two small latero-neurals pass along 

 each side of this same cord. All these longitudinal vessels, 

 of which the first three are most important, are parallel 

 with one another ; the first three meet in an anterior net- 



