STRUCTURE OF LOB-WORM 



261 



along the whole mid-dorsal line of the gut a contractile dorsal vessel, 

 which carries blood fonvards from the seven posterior gills, etc. 

 Connected with this by capillaries, there is below the gut an equally 

 long, feebly contractile ventral vessel, which carries blood backwards 

 to gills, nephridia, etc. Around the gastric region of the gut there is an 

 elaborate plexus of blood vessels, which communicate by two lateral 

 vessels with the paired heart. There are also two sub -intestinal vessels 

 between the ventral vessel and the gut ; these lead through the plexus 

 into the lateral gastric vessels, and thus into the hearts. These organs 

 lie just behind the oesophageal glands, and consist on each side — [a) of 

 a thin-walled auricle, an expansion of the lateral gastric vessel ; and (6) 

 of a muscular ventricle, which drives the blood into the ventral vessel. 



Fig. 141. — Cross-section of Arenicola. — After Cosmovici. 



E., Epidermis ; cm., circular muscles ; l.m., longitudinal muscles ; 

 b.c, body cavity ; gl., gill ; s., setae ; n.p., nephridial pore ; 

 a.br., a:^erent branchial ; e.br., efferent branchial ; n., ventral 

 nerve-cord, with blood vessels above ; d.v., dorsal vessel ; l.v., 

 lateral vessel ; s.i.v., sub-intestinal vessels ; v.v., ventral vessel ; 

 g-, gut. 



Like the sub-intestinals, the dorsal vessel communicates with the heart 

 only indirectly through the gastric plexus. The ventricle contains a 

 spongy " cardiac body," which probably prevents regurgitation from 

 the ventral vessel. 



From the ventral vessel arise afferent branchial vessels to gills, 

 nephridia, etc. From the seven posterior gills efferent branches enter 

 the dorsal vessel ; while those from the six anterior gills join the sub- 

 intestinals. Each efferent vessel gives off a branch to the skin, while 

 the dorsal and sub-intestinal vessels give off numerous branches to the 

 gastric plexus on the gut. 



Respiratory system. — There are thirteen pairs of gills, 

 on the seventh to the nineteenth bristle-bearing segments. 



