CHAP. V.] THE NERVOUS SYSTEM OF VERMES. 



91 



The lip is regarded as an organ of touch, but it is 

 equally probable that it is capable of receiving more 

 special impressions representing rudimentary tastes. The 

 separation between these modes of sensibility in such low 

 organisms is probably somewhat indefinite. 



The double ventral cord has a fibrous structure along 

 its upper surface, whilst below there is 

 an irregular stratum of ganglion cells. 

 These cells are more abundant about 

 the centre of each body- segment, so 

 that their aggregation gives rise to a 

 series of rudimentary ganglia in these 

 situations. From every one of the 

 ganglionic swellings two nerves are 

 given off on each side ; whilst a third 

 pair issues from the cord itself, just 

 anterior to the swelling, and is dis- 

 tributed along the anterior boundaries 

 of the segment. In Serpula, one of the 

 small tube-dwelling marine worms, the 

 ventral ganglia are also very minute, 

 and those of the two sides, together 

 with the ventral cords, lie some distance 

 apart, and are connected by a series of 



COmmiSSUreS (fig. 32, 6). In thlS dlS- 



position of the great nervous cords we 



have something intermediate between pfcageai ganglia; 6, sub- 



,i -i , t ... .1 -VT cesophageal ganglia ; b' one 



their lateral position in theNemerteans, of gangLnated cords; , 

 and their contiguous mid-ventral posi- motor buccal nerves ; * tac ~ 



, tile nerves. 



rion in the .beech and the Earthworm. 



As in the latter, so in Serpula, the afferent nerves entering 



the brain (t) seem to be in the main tactile. 



The cesophageal ganglia in the Earthworm are, propor- 

 tionately to the rest of the nervous system, much smaller 



FIG. 32. Nervous 8ys- 

 tern of Serpula contort*- 



