EARTHWORM 



233 



a current which draws sohd waste particles from the coelomic fluid into 

 the nephridial tube and cilia in the tube pass these onward and out of 

 the body. Glands in the coiled tube also take nitrogenous waste matter 

 from the blood and add it to the liquid in the nephridium, which results 

 in its elimination. 



272. Musculature and Locomotion. — The muscles of the earthworm 

 are arranged in two layers. An outer layer of circular muscle fibers 

 just below the skin forms a continuous sheet about the body. An inner 

 layer of longitudinal muscles is arranged in several bands — two dorsal, 

 two ventral, and a lateral one on each side between the double rows of 

 setae (Fig. 135). 



5uprapharyn^eaf 



ofangJi'or? 



Moufh 



Bucccr/ 

 rav/fy 



ffcrr;£rJ/on 



Fig. 



Ci'rcumpharyp^rea/ if'^'^'^'ftJ'-' "Se^tnei^faf 

 connecr/ve gian^fi'on 



138. — Anterior portion of the nervous system of an earthworm. Lateral view. 

 {From Hess, in Jour. Morphology, vol. 40.) Segments numbered in roman. 



Locomotion is carried on by extension, contraction, and flexion of 

 the body due to the muscle layers and also by the use of the setae. 

 Muscles attached to the latter structures serve to retract them within 

 their sheaths in the skin or, when they are protruded, to move them 

 either forward or backward. Accordingly, acting as so many little 

 levers, the setae serve to propel the body in either direction within the 

 burrow. 



273. Nervous System. — The nervous system in the earthworm con- 

 sists of segmentally arranged, paired ganglia and a nerve trunk, which 

 together form a central nervous system, and of peripheral nerves which 

 distribute fibers to various parts of the body. The central nervous 

 system begins anteriorly in a pair of suprapharyngeal ganglia (Fig. 138) 

 fused into a bilobed mass which is located in the third metamere and 

 above the front end of the pharynx. From these ganglia two nerve 

 trunks known as circumpharyngeal connectives pass around the pharynx, 

 one on each side, to unite below in the same metamere, forming a ventral 

 nerve cord. Upon this is a bilobed dilatation in the fourth metamere 

 formed by the subpharyngeal ganglia. The ventral nerve cord continues 



