STRUCTURE OF THE EARTHWORM. 191 



cord. As the segments are very short, the limits of the 

 successive pairs of ganglia are not very evident, especially 

 in the anterior region, but they are plain enough on a small 

 portion of the cord examined with the microscope, when it 

 may also be seen that each ollhe pairs of ganglia gives off 

 nerves to the walls of the body. Anteriorly, just behind 

 the mouth, the halves of the cord diverge and ascend, 

 forming a ring around the pharynx. They unite above 

 in two dorsal or cerebral ganglia. These form the earth- 

 worm's " brain," and give off nerves to the adjacent pre-oral 

 segment or prostomium, on which are numerous sensitive 

 cells. These, coming in contact with many things, doubtless 

 receive impressions, which are transmitted by the associated 

 nerves to the "brain." As Mr. Darwin observed that 

 earthworms seized hold of leaves in the most expeditious 

 fashion, taking the sharp twin leaves of the Scotch fir by their 

 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. 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 earthworm'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, 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 contents. These " giant fibres," which do not seem to 

 be nervous, but are rather supporting elements, have been dignified by 

 the name of neurochord. 



