168 



PHYSIOLOGY 



with the wave-like contraction of the dorsal vessel cause 

 the blood to circulate. 



A Segment. — Corresponding to the external rings are 

 thin partitions called septa or dissepiments that separate 

 the body into little compartments called segments. Ex- 

 cept for those in the anterior end that contain the re- 

 productive organs and the aortic arches, the segments 



Fig. 97.— Cross section of the earthworm, a.c, cavity of the alimentary canal ; 

 c, cuticle ; cm, coelom; cm., circular muscles ; c.v, circular vessel ; d.v, dorsal 

 vessel ; l.m., longitudinal muscles ; n. c, ventral uerve-chain ; s, seta ; s.i.v, sub- 

 intestinal vessel; a.m., muscle connecting seta of the same side; ty, typh- 

 losole. (From Sedgwick and Wilson.) 



are alike (Fig. 97). Each one contains a section of the 

 longitudinal muscles, a circular muscle, two pairs of 

 seta?, a ganglion with its pair of nerves, a circular blood 

 vessel with sections of the longitudinal vessels, a section 

 of the alimentary canal and a pair of nephridia. 



