LUMBRICUS TERRESTRIS 



167 



vessels in the tenth segment from the oesophagus, and ends in 

 front by breaking up into branches which supply the pharynx. 

 In each of the segments 7-1 1 it gives off a pair of large contractile 

 vessels or pseudo-hearts. These encircle the oesophagus and join a 

 ventral or subintestinal vessel which hangs by a mesentery 

 below the gut. In the pseudo-hearts the blood flows downwards 

 from the dorsal to the ventral vessel, and in the latter it flows 

 backwards and forwards from the region of the hearts. From the 

 ventral vessel the blood passes by a series of small vessels to the 

 intestine, and by parietal vessels to the nephridia and to the body- 

 wall. From these organs it is returned along various paths to 

 the dorsal vessel. Among 

 the subsidiary vessels are 

 a subneural and two lateral 

 neural vessels, in which the 

 blood flows backwards, and 

 dorso-subneural vessels, a 

 pair in each segment of the 

 intestinal region of the 

 body, which carry blood 

 to the dorsal vessel from 

 the subneural vessel, the 

 nephridia, and the body- 

 wall. The main blood vessels 

 of the earthworm cannot 

 be distinguished into art- 

 eries and veins, but their 

 ends are joined by capil- 

 laries. The dorsal vessel 

 and the pseudo-hearts are 



provided with valves which keep the blood flowing in the proper 

 direction. 



NERVOUS SYSTEM 



The earthworm has a well-developed central nervous system 

 (Fig. 118), which consists of (i) a pair of suprapharyngeal ganglia, 

 rounded bodies lying above the mouth, and sometimes known 

 together as the brain, (2) two slender circumpharyngeal com- 

 missures running from these round the pharynx, and (3) a ventral 

 nerve cord which starts from the commissures between the 

 third and fourth somites and runs the whole length of the body 



s.n^.v s.i.v. 



Fig. 117. — A diagram of a transverse section 

 of the earthworm in the intestinal region 

 to show the arrangement of the blood 



vessels. 



ejf.n.v., efferent vessel from nephridium ; l.n.v., lateral 

 neural vessel ; nph., nephridium ; other lettering as 

 in Fig. 115. 



