302 BIOLOGICAL LECTURES. 



sensillae in annelids are less sensitive than those in aquatic 

 vertebrates. We have, then, no reliable test of sensibility 

 either in the structure of the sense organ, in the rank of the 

 organism, or in its intelligence. We have to depend upon the 

 response to stimuli, and, remembering that this may be decep- 

 tive, observe and experiment under conditions that insure free 

 behavior. 



No one who has never come into close communion with the 

 lower animals can begin to appreciate the delicacy and effi- 

 ciency of their sensory apparatus. We take up the earthworm 

 and, as we see no eyes, we conclude that it cannot see. A 

 little experiment shows that it is extremely sensitive to light, 

 and further study of its structure reveals unpigmented eyes 

 lying beneath the skin, and the whole surface thickly set with 

 minute delicate tactile sensillae. Even Amphioxus> so long 

 reputed to have no visual organs, turns out to have such 

 organs from end to end, imbedded in its spinal cord. I have 

 before called attention to the highly sensitive organization of 

 Clepsine and its allies. In the very lowest organisms, plant 

 and animal alike, where special visual organs do not exist, the 

 living protoplasm has, as has been demonstrated in many ways, 

 a keen sensibility to light, so that one might look upon the 

 whole organism as fulfilling the light-perceiving function. 



g. Orientation through the Dermal Semitic?. 



Necturus, as before remarked, has a very keenly sensitive 

 organization. The skin is richly provided with sense organs, 

 which terminate at the surface in very short, fine hairs, invisible 

 to the naked eye. These organs, which are of the same nature 

 and function as the dermal sensillae in Clepsine and in so 

 many other aquatic animals, are sensitive to slight vibrations 

 in the water that are far beyond the reach of any of our sense 

 organs, and they are the main reliance, both in avoiding enemies 

 and seeking prey. 



It is interesting to see how little the eyes are depended upon 

 in finding a piece of meat. A bit dropped in front of a young 

 Necturus receives no attention after it reaches the bottom. 



