fe) 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 wesceral nervous 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. 
Hl) Though these constitute a special group 
PS ventrai Al quite distinct from Annelids, they are de- 
scended from common ancestors, some of 
' the primitive characters of which they have 
Fig. ror2.—Front Part of Cen» probably retained, one being the possession 
tral Nervous System in two Marine : . . 
Annelids, enlarged. In A the two Of a Strong lateral nerve on either side, in- 
Lath theneiein ce stead of a double ventral cord (fg. tora). 
ee ae 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- 
popA).—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. 
A Brain B 
