STRUCTURE OF THE EARTHWORM. 169 



united base, we may credit the earthworms with some power 

 of profiting by experience ; moreover, as they deal deftly 

 with leaves of which they have no previous experience, we 

 may even charitably grant them a modicum of intelligence. 

 From the nerve-collar uniting the dorsal ganglia with the 

 first pair on the ventral cord, nerves are given off to the 

 pharynx or gut, forming what is called a " visceral system." 

 The earthworm has no special sense-organs, but we have just 

 mentioned sensitive cells, which are particularly abundant 

 on the head end of the worm. By them the 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. 



Two facts in regard to minute structure deserve attention. 

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

 or ganglia, as they are in Arthropods, occur diffusely along 

 with the nerve-fibres throughout the course of the cord. 

 Along the dorsal surface of the ventral nerve-cord there run 

 three peculiar tubular fibres, with firm walls and clear con- 

 tents. These "giant fibres," which do not seem to be nervous, 

 but are rather supporting elements, have been dignified by 

 the name of lieurochord, and ingeniously compared with the 

 notochord of Vertebrates. 



Alimentary System. — Earthworms eat the soil for the sake 

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

 because they must swallow as they tunnel. In eating they 

 are greatly helped by the muscular nature of the pharynx, 

 whence the soil passes down the gullet or cEsophagus, first 

 into a swollen crop, then into a strong-walled grinding 

 gizzard, and finally along a long digestive and absorptive 

 stomach-intestine. On the gullet are three pairs of ceso- 

 phageal or calciferous glands — the products of which are limy 

 and able to affect the food chemically, probably counteracting 

 the acidity of the decaying vegetable matter. The long 

 intestine has its internal surface increased by a dorsal fold, 

 which projects inwards along the whole length. In this fold, 

 and all over the outer surface of the gut, yellow-cells are 

 thickly crowded, There is no warrant for calling these 

 hepatic or digestive. Structurally they are pigmented cells of 

 the peritoneal epithelium, which here, as in most other 



