108 PARKER— THE RELATIONS OF THE 



its environment. To this question much of the evidence of recent 

 years seems to be shaping an afifirmative answer. 



One of the newer Hues of evidence touching on this point has to 

 do with sense organs. These organs are usually regarded as bodily 

 parts concerned with providing us with the elements of information 

 as to the world about us. They are thus intimately associated with 

 our central nervous activities. But they are known to occur in many 

 lowly organized animals, such as the jellyfishes and the like, in which 

 there are no central nervous organs appropriate for such information. 

 In these animals the nervous impulses from the so-called sense organs 

 pass directly to the muscles without first making their way through a 

 central nervous organ. They serve merely as a means of exciting 

 muscular activity and are concerned in no way at all, so far as one 

 can judge, with sensations. Their action is comparable to that of 

 our eye, which, when brightly illuminated, so responds that the muscu- 

 lar sphincter in the iris contracts, thus reducing the size of the pupil. 

 With us sense organs have two functions. They deliver impulses 

 that excite muscles to action, as in the instance just given, and they 

 deliver impulses that serve our central organs in an informing way. 

 Of these two functions only the first is possessed by many of the 

 lower animals. Hence it is without doubt the more primitive, for 

 the second function could not have arisen before the development of 

 a central nervous organ, a part which, as already intimated, is absent 

 from many simple animals. 



To the older naturalists the presence of a sense organ was suffi- 

 cient grounds for assuming that the animal experienced sensations 

 characteristic of that organ. Thus the recognition of eye spots in 

 jellyfishes was supposed to justify the opinion that these animals 

 could see. But from the standpoint of the more recent work the 

 presence of such an organ merely means that the animal is especially 

 responsive to light, not that it has the sensations of sight, for the 

 nervous strands from the eye spot in the jellyfish lead directly to the 

 muscles and not to a central nervous organ. Hence the so-called 

 sense organs of the lower animals, since they are in no necessary way 

 concerned with sensations, are more correctly designated as receptors 

 in consequence of their relation to the stimulus. 



