NERVOUS SYSTEM OF EARTHWORM 249 



to last, but compactly united into what to unaided eyes 

 seems a single 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 micro- 

 scope, when it may also be seen that each of the 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 round the pharynx. They unite 

 above in two dorsal or cerebral ganglia, which are situated 

 in the peristomium or first ring, and not, as in Polychaetes, 

 in the prostomium. These form the earthworm's " brain," 

 and give off nerves to the adjacent pre-oral lobe or pro- 

 stomium, 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 

 grant them a modicum of intelligence. From the nerve- 

 collar uniting a dorsal ganglia with the first pair on the 

 ventral cord, nerves are given off to the pharynx and gut, 

 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. 



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. 



