PHYLUM ANNULATA 



477 



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 (Serpiilaaud 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 cesophageal 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 



set 



Fi<;. o7>. Saccocimis, transverse section, to show the position of the nerve-cords, dors. vess. 

 dorsal vessel ; int. intestine ; ne. co. nerve-cord ; set. setae. (After Fraipont.) 



of the Chsetopoda the cerebral ganglion and the ventral chain are 

 separated from the epidermis by muscular layers ; in some, how- 

 ever, the ventral chain is in contact with the epidermis, and in 

 certain primitive or aberrant forms, the Archi-Chaetopoda (Fig. 

 375) and Sternaspis, the cerebral ganglion is in close union with 

 the epidermis ; in these also the ventral cord is not segmented 

 into ganglia. Running longitudinally through the ventral cord in 

 many forms are certain giant fibres of very large size ; though these 

 may have rather a skeletal than a nervous function, they are simply 

 greatly enlarged and modified nerve-fibres. Nerve-cells may be 

 confined to the ganglia, or may be distributed over the entire sur- 



