x PHYLUM ANNULATA 465 



system. Sometimes the blood is colourless : very commonly it 

 is bright red in colour, owing to the presence of haemoglobin, 

 which is not confined to the corpuscles, but is dissolved in the 

 plasma. In Serpula and its allies the blood is bright green, owing 

 to the presence of a green colouring matter, which has an affinity 

 for oxygen similar to that possessed by haemoglobin. 



The chief blood-vessels are usually dorsal and ventral longi- 

 tudinal trunks. These are connected together by metamerically 

 arranged transverse branches. In some of the Cryptocephala the 

 dorsal vessel is not present in the greater part of the length of the 

 body, its place being taken by a peri-intestinal sinus or a plexus of 

 vessels lying in the wall of the alimentary canal. This gives off in 

 front a short thick-walled dorsal vessel or " heart." The movement 

 of the blood is effected in most instances by peristaltic contractions 

 of the dorsal vessel, or of a peri-intestinal sinus or plexus, or of the 

 ; ' heart " given off by the latter anteriorly, which have the effect of 

 driving the blood from behind forwards. In some instances, as in 

 the Earthworms and some Cryptocephala, specially dilated lateral 

 vessels are contractile, and by their pulsations bring about the 

 circulation of the blood through the system of vessels. Plexuses 

 of fine capillary vessels in the integument of various parts frequently 

 aid in respiration, and are particularly well developed in certain 

 forms in which definite organs of respiration are absent. 



The nervous system consists of a cerebral ganglion or brain 

 and a double ventral chain of ganglia. The cerebral ganglion is 

 distinctly bilobed, and may be looked upon as composed of two 

 intimately united ganglia. It is almost invariably situated in the 

 prostomium, though placed a little further back in the Earth- 

 worms ; it gives off branches to the eyes and tentacles. From it 

 there run backwards and downwards the paired cesophageal con- 

 nectives, which embrace the anterior part of the alimentary canal 

 between them, and below join the anterior end of the ventral chain 

 of ganglia. The latter always exhibits indications of being made 

 up of two lateral halves in the double character of the connecting 

 commissures and frequently of the ganglia themselves. One of 

 these double ganglia occurs in each segment, and from it a number 

 of nerves pass out to the various parts of the segment. In certain 

 Cryptocephala (Serpula and others) the two halves of the chain are 

 separated from one another by a wide space, across which trans- 

 verse commissures pass between the ganglia. Connected with the 

 cerebral ganglia, or with the oesophageal connectives, or with both, 

 there is a system of delicate stomatogastric nerves passing to the 

 walls of the anterior part of the alimentary canal. In the majority 

 of the Chsetopoda the cerebral ganglion and the ventral chain are 

 separated from the epidermis ; in some, however, the ventral chain 

 is in contact with the epidermis, and in certain primitive or aberrant 

 forms, the Archi-Chsetopoda (Fig. 381) and Sternaspis, the cerebral 



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