TYPE ANNELIDA. 221 



The nervous system consists of a supraoesophageal gau- 

 glioii (Fig. 104, ce) of a somewhat complicated structure lying 

 in the anterior portion of the body. In jEolosoma it is con- 

 nected with the hypodermis (i.e., the ectoderm), but in most 

 forms it lies upon the anterior part of the digestive tract, quite 

 separate from the hypodermis. It differs in position from 

 the corresponding ganglion of the Polychseta in that it is not 

 usually situated in the anterior nietamere or prostomium, but 

 has passed farther back and may lie in the second, third, or 

 fourth metarnere or even more posteriorly. It sends off nerves 

 to the sensitive prostomium and gives rise to two commis- 

 sures which pass backwards and downwards on either side of 

 the pharynx to unite with the subcesophageal ganglion (so], 

 which, like the brain, is formed of two more or less fused lat- 

 eral masses, each of which in many forms shows indications of 

 being compound and formed by the fusion of two or more 

 ganglia (Lumbricus}. To this there succeeds in each nieta- 

 mere a pair of ganglia each of which is united to its prede- 

 cessor and successor by a pair of connecting cords, the whole 

 ventral cord (ri) so produced having a characteristic ladder- 

 like arrangement. Usually the connecting cords are closely 

 approximated, and the same may be the case with the gan- 

 glion pairs, the whole being ensheathed in connective tissue 

 so that the cord seems to be single. From each pair of gan- 

 glia in Lumbricus three nerves pass out on each side, the two 

 posterior ones being closely related so as to appear to be one. 

 Nerves especially connected with the digestive tract, the 

 stomatogastric nerves, seem to be present, but their distribu- 

 tion and connections have not yet been thoroughly studied. 

 In some aquatic forms a lateral nerve imbedded in the hypo- 

 dermis and united anteriorly with the supraoesophageal 

 ganglion runs along the lateral line between the two rows of 

 sefoe, recalling the lateral-line nerve of the CapitelUdce among 

 the Polychseta. 



Sense-organs of various kinds are present. Tentacles are 

 absent throughout the group, and in only a few forms (Nais) 

 do eyes, consisting of pigment-spots imbedded in the hypo- 

 dermis, occur. Ciliated depressions at the side of the pro- 

 stomial segment occur in ^EoLosoma and a few other genera, 



