ARENICOLA. 227 



third diaphragm the gut swings freely until the beginning 

 of the tail region, in which there are many septa. 



Vascular system. The blood has a bright red colour, and is rich 

 in haemoglobin. It flows in a very elaborate system of blood vessels, in 

 regard to the details of which there is still some uncertainty. There is 

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

 which carries blood forwards 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 (b) 

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

 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. 

 Each is a tuft of hollow thread-like branches, through the 

 thin walls of which the red blood shines. The afferent 

 branches to the gills all come from the ventral vessel ; the 

 first six efferent vessels from the gills open into the sub- 

 intestinals ; the posterior seven open into the dorsal vessel. 

 As the papillae on the proboscis are hollow and contain 

 vessels, they are. doubtless of respiratory significance. 

 Indeed, the gills may be regarded as exaggerated papillae. 



Excretory and reproductive systems. In the anterior 

 region, in segments 4-9, there are six pairs of nephridia, 

 of which the foremost seems in process of degeneration. 

 Each consists of three parts a funnel opening into the 

 body cavity, a glandular portion, and a bladder com- 

 municating with the exterior. 



The sexes are separate and similar. The reproductive 

 organs are very simple, and arise by proliferation of the 



