Sensory Discrimination : Vision 1 39 



dwelling with a peculiar hesitating flight; afterwards, says 

 von Buttel-Reepen, they become "frecher." He declares 

 that when attempting to alight before a foreign hive they are 

 often driven off by the rightful occupants before their odor 

 can have been noticed, and ascribes this reaction to the sight 

 of their hesitating method of approach. On the other hand, 

 when a broodless stock joins itself to one that has a brood, the 

 latter is induced to receive them peacefully because of their 

 assured manner (72). 



The majority of bee students incline to the belief that bees 

 are guided back to their hives from long flights by visual 

 memory, though the phenomena are not easy to explain. 

 Solitary wasps, it seems highly probable from experiments, 

 find their nests by sight ; but this subject will be more fully 

 discussed in Chapter XI. 



50. Vision in Amphioxus and in Fish 



The vertebrate eye differs in origin and in structure from 

 any form of invertebrate eye, the most striking difference in 

 structure being perhaps the situation of the pigmented layer 

 of the retina behind the nerve-fibre layer, which is respon- 

 sible for the existence of the blind spot where the trunk of 

 the optic nerve breaks through the retinal layer. Another 

 point of unlikeness consists in the fact that the invertebrate 

 optic nerves do not cross on their way to the brain, while in 

 the vertebrates there is either total or partial crossing of the 

 fibres. In both the vertebrate and the simple invertebrate eye 

 the image is formed by means of a lens, although Nagel has 

 suggested that the function of the lens in the lower forms of eye 

 is rather to collect the light than to produce an image (293). 



The reactions of Amphioxus to light offer as chief evidence 

 that they are accompanied by a specific sensation quality 

 the fact that they may be fatigued independently of other 



