7 6 



ZOOLOGY. 



structure appear to be much as described for the otocysts. In some of the 

 lower animals, as insects, there are also external vibratile hairs which are 

 believed to be auditory (Fig. 40). 



FIG. 40. 



FIG. 40. Antenna of Male Mosquito (.Culex pipiens). By J. W. Folsom. 



Questions on the figure. Compare with the antennae of a female (see 

 Fig. 63). What are the differences between the head of the male and 

 female mosquitoes? What is believed to be the function of these plumose 

 antennas? What are the evidences for this view? 



109. Sight. There are three distinct facts to be noted with respect to 

 visual sensation in the higher forms of animals: the perception of light, 

 the perception of color (. e., light of different wave-frequency) and the 

 formation of images of external objects. It has already been seen (20) 

 that protoplasm is sensitive and responsive to light without any special 

 organs. The simplest visual organs found in multicellular animals con- 

 sist merely of epithelial cells containing pigment in which changes are 

 wrought apparently by the action of light (Fig. 41, a). These changes 

 affect the nerve fibres associated with the pigment cells and thus the 

 central nervous organ. Such eyes are capable only of giving knowledge 

 of the intensity or, if properly constructed, of intensity and direction of 

 the light and do not form an image of external objects. There are 

 several types of image-forming eyes in the animal kingdom. The most 

 familiar of these is the "camera eye" of vertebrates, so called because 



