190 SEGMENTED WORMS OR ANNELIDA. 



Skin and Bristles. 



Outermost lies the thin cuticle, on which intersecting lines 

 produce interference of light and irridescence. Like any 

 other cuticle, it is produced by the cells which lie beneath, 

 and it is perforated by the apertures previously mentioned. 

 The epidermis clothing the worm is a single layer of cells, 

 of which most are simply supporting or covering elements, 

 while many are slightly modified, as glandular or mucous 

 cells, and as nervous cells. As the latter are connected with 

 afferent fibres which enter the nerve cord, the skin is 

 diffusely sensitive. 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. 



A'luscular System. 



The earthworm moves by the contraction of muscle 

 cells, which are arranged in hoops underneath 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, 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. 



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 



