214 PHYLUM ANNELIDA. 
segments, and on the outermost portions of the excretory 
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 Polychetes, 
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 
