214 PHYLUM ANNELIDA . 



segments, and on the outermost portions of the excretoiy 

 tubes. 



Unlike the leech, the earthworm has a very distinct body 

 cavity, through the middle of which the gut extends, and 

 across which run the partitions or septa incompletely 

 separating successive segments. In this cavity there is 

 some fluid with cellular elements, of which the most 

 numerous are yellow cells detached from the walls of the 

 gut. Possible communications with the exterior are by 

 the dorsal pores, and also by the excretory tubes, which 

 open internally into the cavities of the segments. 



Nervous system. Along the middle ventral line lies a 

 chain of nerve-centres or ganglia, really double from first 

 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 the 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 



