OBSERVATIONS ON UNICELLULAR FORMS 1 09 



structure, and moving toward the shadow would decrease 

 it still more. 



There is consequently nothing in connection with the 

 observations of Engelmann which contradicts the idea that 

 the eye-spot in Euglena functions as an opaque screen, and 

 that there is a bit of protoplasm which is highly sensitive 

 to changes in light intensity in close contact with it. The 

 hyaline protoplasm at the anterior end condenses the light 

 so that it is most intense in the neighborhood of the eye- 

 spot. This can be seen in Euglena in direct sunlight. It is 

 however much more marked in Chlamydomonas and the 

 zooids of Eudorina and Pandorina (Figs. 17 and 21). If 

 the light is thus actually focused on the most sensitive 

 structure of the organism it is easy to see how changes in 

 the general direction of the rays could produce marked 

 changes of intensity on this structure. Aside from acting 

 as an opaque screen the eye-spot may, of course, as already 

 stated, also function as an absorptive background. 



In Trachelomonas hispida the eye-spot is situated very 

 near the middle of the anterior end (see Fig. 16). If it 

 functions by shading the sensitive portion or by absorbing 

 the rays in this form it is highly probable that the sensitive 

 portion consists of a minute structure situated very near 

 it, perhaps in the hollow of the concave surface. In some 

 of the forms however the location of this structure seems 

 to show that it does not function as a screen. In Vol vox, 

 Pandorina and Eudorina the eye-spots are situated on the 

 outer posterior surface of the zooids. It is difficult to see 

 how they could function as screens in these forms (see Fig. 

 21). The same difficulty is encountered in some forms of 

 Chlamydomonas in which this structure is situated near 

 the posterior end (see Fig. 17). It does however not seem 

 necessary to assume that the eye-spot functions precisely 

 the same in all forms. While it may function both as an 

 opaque screen and as an absorptive background in Euglena, 

 it may possibly function only by absorbing light rays in 

 Volvox and Chlamydomonas, 



