DEVELOPMENT OF MIND IN ANIMALS 125 



body, somewhat like our spinal cord except that it lies under 

 the digestive system instead of above. This worm is seg- 

 mented; and in each segment there are subsidiary ganglia or 

 "local brainlets," giving off lateral connectives, all on a bi- 

 lateral plan. Annelid worms in their symmetry and body 

 plan are broadly similar to the basic vertebrate plan, al- 

 though upside down. Many students of biology have seen 

 in the anneUd a form which branched off from the ancestral 

 type leading to man. The theory still has some supporters 

 although little direct evidence is available. It is clear, how- 

 ever, that the arthropod (insects, spiders, etc.) and the anne- 

 lid are close relatives and definitely come from a common 

 stock. The insect is more advanced, but with a basic simi- 

 larity in the nervous system: a two-part forebrain and a 

 nerve-trunk with a lesser number of local ganglia. We w^ill 

 see later how terrifically talented in instinctive behavior 

 some of the arthropods become. The annelids, too, are not 

 without some capacities, both in learning and instinctive be- 

 havior. 



Learning capacity in the earthworm was first studied by 

 Yerkes, who placed these animals in a narrow tube made in 

 the shape of a T so that they would have to creep toward the 

 crossbar. When the earthworms arrived at the parting of 

 the ways, they could go either right or left. To the right 

 they received an electric shock sufficiently strong to punish 

 them mildly; to the left they escaped without punishment. 

 At first the choice was made at random, but after a number 

 of trials the individuals began to avoid the right turn. Fi- 

 nally, after repeated trials the choice was nearly always to 

 the left to escape punishment. Yerkes then moved the elec- 

 trodes from the right tube to the left, and the experimental 

 animals had to unlearn the former lessons. The reversal was 

 learned faster than the original habit, meaning, perhaps, the 

 development of prepotent responsiveness to the effective 

 cue and the association of perception with motor response. 

 Yerkes removed the ganglia around the mouth, and even 



