STRUCTURE OP ARENICOLA. 



205 



or ventricles, each of which receives blood from the dorsal 

 vessel, from a sub-intestinal vessel, and from a lateral vessel, 

 and drives blood into the ventral vessel. Each of the lateral 

 vessels before entering the heart expands into a kind of 

 auricle. 



The longitudinal vessels are all connected by transverse 

 branches. From the ventral vessel arise afferent branchial 

 vessels. 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 



Fig. 67. — Cross section of Arenicola. (After CosMOVici.) 



E. Epidermis; c.//?., circular muscles ; /./«., longitudinal muscles; 

 b.c.-, body cavity ; ^/. , gill; ^., setae; n.p.^ nephridial pore; a,br,, 

 afferent branchial ; ^./r., efferent branchial; ?;., ventral nerve-cord 

 with blood vessels above; d.zK, dorsal vessel; /.-'., lateral vessel; 

 s.i.v., sub-intestinal vessel ; z'.z'., ventral vessel ; ^., gut. 



a branch to the skin, while the dorsal and sub-intestinal 

 vessels give off numerous branches to the walls of the gut. 

 It seems that the flow of the blood is not always quite the 

 same. 



Respiratory System. 



There are thirteen pairs of gills. Each is a tuft of 

 thread-like branches, through the thin walls of which the 



