10 



NERVOUS SYSTEM AND SENSE-ORGANS 



It has already been stated that one of the duties of a nervous 

 system is to correlate the organs of the body itself, and even in 

 an earth-worm there is a special arrangement for controlling the 

 digestive organs, and consisting of nerves which run from the 

 sides of the nerve-ring to the gut, and branch out in a complex 

 way, the branches swelling here and there into extremely minute 

 ganglia. This arrangement is called the visceral nervoiis system^ 

 and, like the ventral cord, is subject to the control of the brain. 



Before leaving segmented worms one feature in the nerve- 

 cord is deserving of notice. It is distinctly of double nature, and 



in some cases its two longitudinal halves 

 are widely separated (fig. 1012). In the 

 evolution of this type of nervous system it 

 is probable that each side of the body de- 

 veloped and was regulated by its own longi- 

 tudinal nerve-cord, and this is actually the 

 arrangement found in the curious unseg- 

 mented forms known as Nemertine Worms. 

 Though these constitute a special group 

 quite distinct from Annelids, they are de- 

 scended from common ancestors, some of 

 the primitive characters of which they have 

 Fig. ioi2.-Front Part of Cen- probably retained, one being the possession 

 of a strong lateral nerve on either side, in- 

 stead of a doubl e ventral cord (fig. 1013). 

 N - R - Such an arrangement is not a desirable one, 

 for it means imperfect correlation between 

 right and left sides of the body. The ventral cord of an Annelid 

 has quite likely been derived from lateral cords of the kind, which 

 have migrated downwards and come into more or less intimate 

 relation with one another in the interests of centralization. 



NERVOUS SYSTEMS OF JOINTED-LIMBED INVERTEBRATES (ARTHRO- 

 PODA). There can be little doubt that the members of this huge 

 group have sprung from ancestors which resembled Annelids in 

 many respects. But they have specialized in various ways, partly 

 as the result of centralizing tendencies which have resulted in 

 increased complexity of structure, associated with very perfect 

 adjustment to surroundings. The body, instead of being greatly 

 elongated and made up of a large number of rings or segments, 

 is comparatively short, and composed of relatively few segments. 



together but not fused, 

 nerve-ring. 



