DEVELOPMENT OF MIND IN ANIMALS 127 



activity of the viscera. Throughout this great phylum there 

 is considerable modification, but the basic plan is always 

 present. Nissen summarizes literature which shows that land 

 snails learned to master the T maze in experiments some- 

 what similar to those performed on the earthworm, in faster 

 time than the latter did. In another conditioning test a pond 

 snail was given double stimulations with lettuce touching 

 the mouth (causing the mouth to open) and pressure on the 

 foot (inhibiting the opening of the mouth). At first the 

 pressure on the foot was prepotent over the lettuce stimulus 

 to the mouth. After about 250 trials, the mechanical stimulus 

 to the foot resulted in the snail opening its mouth even 

 when the lettuce stimulus was omitted. Experimental extinc- 

 tion could be obtained in about 1 2 trials, but reconditioning 

 was more quickly established than in the original training. 

 Differential conditioning has been observed in other mol- 

 luscs. 



Arthropods, as has been mentioned, have basically an an- 

 nelid nervous system. Insects, crabs, and others in this phy- 

 lum draw the middle segmental ganglia together into a large 

 ner\^ous mass located between the legs, forming a mid-brain 

 which is almost as large as the forebrain. There is propor- 

 tionately more nervous tissue and greater complexity of or- 

 ganization in the higher arthropods than in any other in- 

 vertebrate group, and the arthropods very definitely show 

 this in their behavior, both in learning and in instinct. Ants, 

 for instance, have been repeatedly found to have greater 

 learning capacity than any other invertebrate. In fact, Nis- 

 sen reports that to some extent they compare in intelligent 

 behavior even with the lower mammals, and that they are 

 undoubtedly superior to lower vertebrates. All the insects, 

 particularly the Hymenoptera (ants, bees, wasps), learn the 

 shortest path through long mazes with many blind alleys, 

 an accomplishment in which they are more proficient than 

 some lower vertebrates. Insects can be taught to overcome 

 and ignore strong instinctive responses. Cockroaches, for 



