VISUAL TISSUES 



227 



Certain unicellular forms, mostly probably plants, have light-perceiving 

 structures. Here a certain region becomes furnished with a disk or 

 cup-shaped pigment spot and a cuticular lens (Fig. 199). In this region 

 the cytoplasm -is more highly sensitive to light. This is primarily a 

 structure for perceiving the intensity of the light. But, as it is always 

 placed eccentric to the long axis of the plant, and, since the plant moves 

 about, rotating on its long axis, it becomes also a device for perceiving 

 direction of light rays. In colonial forms, presenting these so-called 



FIG. 199. Individual of the flagellate, Chlamydomonas reticulata ; e., eye-spot with pigment 

 and lens; nu., nucleus; c.v., contractile vacuole. X 1000. 



stigmata, the stigmata are always arranged with reference to aiding the 

 colony to determine the direction of the light source. 



We can find no homogeneous basis upon which to classify the visual 

 organs of the Metazoa, owing to the way in which the prominent features 

 of these organs are distributed among the various examples. There 

 seems to be very little homology among them based upon a common 

 ancestry, and the same animal will often have two different kinds of eyes 

 on different parts of its body, or even near one another on the same part. 

 We shall therefore consider them in groups that are rough associations 

 of eyes of somewhat the same degree of tissue complexity, or else which 

 belong to closely related groups of animals. Comparisons of these, from 

 the tissue standpoint, form an interesting study. We shall begin our 

 study of the eye of Metazoa with a very simple eye. 



The eye spot of a plecypod mollusk, Solen, as described by Sharp, is 

 probably the simplest true visual organ that has a demonstrable struc- 



