.''ORGANIC EVOLUTION 165 



that segments of the worm are delimited internally by thin 

 transverse membranous partitions or septa, which divide 

 the coelom into a series of compartments, within which 

 certain structures are serially repeated. At the middle of 

 the body these structures are typically seen. The large 

 enteron occupies the center of the body. A slender vessel 

 of bright red color extends along its dorsal side. This is the 

 dorsal vessel, the central part of a blood vessel system. Its 

 color is due to the contained blood, which it drives forward 

 by evident pulsations of its walls. It is joined in every seg- 

 ment by small lateral paired blood vessels, some of which 

 may be traced to the body wall and some to the walls of the 

 enteron. If the septa be cut for a little way on one side, 

 and the alimentary canal be pushed over to the other side, 

 another smaller longitudinal blood vessel, the sub-intestinal, 

 may be seen extended lengthwise beneath it. In this the 

 flow of the blood is toward the rear. In the seventh to 

 eleventh segments of the body there are large paired 

 strongly contractile vessels, the aortic arches, extending 

 downward each side joining the dorsal vessel to the sub- 

 intestinal. 



Nervous system. On the floor of the body cavity beneath 

 the sub-intestinal vessel lies the white nerve cord, from 

 which slender branching nerves arise in every segment. In 

 the foremost segment of the body this cord divides into two 

 commissures which .pass one on either side of the enteron, 

 and reunite above it in a mass of nervous tissue that is the 

 brain or cerebral ganglion. From the brain arise nerves 

 that pass into the prostomium and into the walls of the 

 enteron. 



The food tube )r enteron. This canal is differentiated at 

 its anterior end into a series of organs. Immediately behind 

 the mouth is a muscular pharynx. The circular muscles of 

 its walls contract it, and the copious radiating fibres that 



