EARTHWORM. 215 



animal is made aware of the differences between light and 

 darkness, and of the approaching tread of human feet, not 

 to speak of the hostile advances of a hungry blackbird. 

 The sense of smell is also developed. The afferent or 

 sensory nerve fibres from the nervous cells of the skin enter 

 the nerve-cord and bifurcate into longitudinal branches, 

 which end freely in the nearest ganglia. In this the earth- 

 worm's nervous system suggests that of Vertebrates. 



The nerve cells, instead of being confined to special centres or 

 ganglia, as they are in Arthropods, also occur diffusely along with the 

 nerve fibres throughout the course of the cord, Along the dorsal 

 surface of the nerve-cord there run three peculiar tubular " giant 

 fibres," with firm walls and clear contents. They are probably 

 comparable to the medullated nerve fibres of Vertebrates. 



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 cesophageal glands. Canals from the posterior 

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

 gullet. Their contents are limy, 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," a.nd 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 



