i68 WORMS. 



layer of cells, of which many are glandular, especially in 

 the region of the saddle. In a few species the skin is slightly 

 phosphorescent. The bristles, which are longest on the 

 genital segments, are much curved, and lie in small sacs of 

 the skin, in which they can be replaced after breakage. 



Muscular System. — The earthworm moves by the con- 

 traction of muscle cells, which are arranged in hoops under- 

 neath the skin, and in longitudinal bands more internally. 

 The special muscles about the mouth and pharynx have 

 considerable powers of grasping, while less obvious muscular 

 elements occur in the wall of the gut, in the partitions which 

 run internally between the segments, and on the outermost 

 portions of the excretory tubes. 



The Body-Cavity. — Unlike the leech, the earthworm has 

 a very distinct body-cavity, down 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 

 important 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. 



The 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 microscope, 

 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 around the pharynx. They unite 

 above in two dorsal or cerebral ganglia. These form the 

 earthworm's " brain," and give oif 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 



