SUPPORT, MOTION, AND SENSATION 



357 



epithelial 



jj^torx 



Smell 



This is one of tlio more important organs of special sense. Even 

 aquatic forms have been shown to possess a fairly keen sense of smell. 

 In land forms, the nasal chamber becomes supplied with sensory 

 olfactory cells that are quite primi- 

 tive, or undifferentiated. The in- 

 sects, which in some cases have a keen 

 sense of smell, have the olfactory 

 organs located on the antennae. 

 Loeb performed an experiment that 

 clearly demonstrated the acuteness of 

 this sense in a butterfly, by suspend- 

 ing a female butterfly in a box and 

 then opening the window. In less 

 than half an hour a male butterfly of 

 the same species was nearby. It 

 soon reached the window, flew into 

 the room, and perched on the box. 

 Two other males also came during the afternoon. Their sense 

 of smell no doubt was responsible for their discovery of the female. 

 Man, whose sense of smell is by no means as keen as that of some 

 other animals, can, nevertheless, detect, for example, one part in a 

 million of iodoform. 



.sa^^rtir^g 



.-'srve 

 ^iber 



Olfactory cells. 



Simple Light Receptors 



The reaction of animals to light is one of the most characteristic 

 responses found in the animal kingdom. In the simplest organisms it 

 has been demonstrated that this reaction may be classified as a positive 

 or negative attraction to light. The ability to react to light indicates 

 the presence of cells or tissues in the animal which are photosensitive. 

 Since, in lowTr forms, the response to light may be detected by the 

 manner in which the animal reacts in the presence or absence of light, 

 or in avoiding illuminated areas, it appears probable that there is a 

 more or less direct connection between the photoreceptor cells and 

 the muscles. The responses to light of such animals as the protozoa, 

 hydroids, and earthworms apparently fall into this category, and 

 has led to their being designated as positively or negatively 

 phototropic. Much interesting experimental work has been done 

 along these lines. 



