Nervous Systems 



817 



removed from either type of worm there is no burrowing and the worms are 

 very quiet. The brain, then, is a sensory center, and it normally has an in- 

 hibitory or restraining control over the motor centers in the subesophageal 

 ganglion. 



An example of integrative cephalic dominance which has a simple morpho- 

 logical basis is found in the responses of an earthworm to light. An earth- 



Corpora pedunculatQ 



Palp nerve 



Optic nerves 



Ganglion cell layer 



NEREIS 



B 



Optic ganglion I 

 Optic ganglion II 

 Optic ganglion III 



Lateral glomerulus 



.^j*^ Central body 



Ganglion cell layer 



Oeuterocerebral lobes 



Antennal nerve 



CALOCARIS 



Tritocerebrum 



Corpora pedunculata 



Central body 



Optic ganglion I 

 Optic ganglion II 

 Optic ganglion III 



Antennal glomerulus 



Fig. 306. Sections of brains (supraesophageal ganglia) of several invertebrate animals. 

 A, Nereis; B, the crustacean Calocaris; C, a worker honeybee. From Hanstrom.^** 



worm normally responds negatively to lateral illumination. When the brain 

 is removed the direction of its response is reversed.^^- If the ventral cord is 

 transected a few segments behind the subesophageal ganglion, the anterior 

 tip turns away from the light, whereas the region behind the cut tends to 

 bend toward the light. If one esophageal connective is cut, the worm tends 

 to circus toward the normal side and when illuminated from the intact side 



