202 PHYLUM ANNELIDA. 



forming what is called a " visceral system." The earth- 

 worm has no special sense organs, but there are abundant 

 sensitive cells, especially on the head end. 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. 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. 



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, 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 fibres, with firm 

 walls and clear contents. These " giant fibres," which have been 

 dignified by the name of neurochord, 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. On the gullet 

 are three pairs of oesophageal or calciferous glands the 

 products of which are limy and able to affect the food 

 chemically, probably counteracting the acidity of the decay- 

 ing vegetable matter. 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, the yellow cells are crowded. 



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, we only know that 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. Various 



