ORIENTATION IN COLONIAL ORGANISM 5 



eye-spot and two flagella which are about as long as the colonies 

 are wide (fig. 2). The zooids are slightly elongated and so situ- 

 ated that the longitudinal axes of all are nearly parallel to each 

 other and approximately perpendicular to the flat surfaces of the 

 colonies. The flagella extend from the anterior surface of the 

 colony and the eye-spots are located near their base on the outer 

 surface of the zooids near the anterior end. 



Fig. 2 Free hand sketch of Gonium as seen from one side, z, zooids; e, eye- 

 spot; p, pyrinoid;/, flagella. The eye-spots are situated on the outer surface of 

 the zooids near the anterior end. For details regarding the structure of the eye- 

 spot, see figure 3. 



3. STRUCTURE OF THE EYE-SPOT IN GONIUM, EUDORINA AND 



OTHER FORMSi 



Ever since the days of Ehrenberg ('31) the eye-spots or stig- 

 mata, as they are frequently called, have been looked upon by 

 many as the most primitive eyes. They have consequently 

 been of great interest especially to those concerned with the evo- 

 lution of the visual apparatus in the higher forms. They have 

 been described in many different organisms by various investi- 

 gators. Among these Franze ('93) probably made the most ex- 

 tensive studies. He investigated them in 31 different species. 



Practically all of those who have worked on the eye-spots main- 

 tain that they consist of two essentially different substances, a 



^ This section is the result of histological studies made by Caswell Grave. It 

 is a pleasure to acknowledge my great indebtedness to him for his generous 

 assistance. 



