PHYSIOLOGY OF THE INVERTEBRATA. 347 



differentiated ; and a large number of the lowest organisms 

 possess an ocular spot, which is a differentiated organ having 

 the function of sight. 



In the Protozoa, this organ is chiefly found in the group of 

 jVIonads or Flagcllata, and is generally coloured red. Klebs 

 has studied the structure of these ocular spots in the Euglena)- 

 When one of these organisms is treated with a solution of 

 sodium chloride (i to iCo), the contractile vesicle, which is 

 in close proximity to the ocular spot, dilates enormously, and 

 consequently causes the same thing to occur in the ocular 

 spot itself. By this means Klebs observed that the spot '• is 

 a small discoid or triangular mass, of jagged and irregular 

 outline (see Fig. i); it is formed of two parts: for a base it 

 has a small mass of reticulated protoplasm, and in the meshes 

 of the protoplasm there are small drops of an oily substance 

 coloured red." 



'•What is the physiological significance of these spots? 

 I'ihrenberg considered them as eyes; hence the name Uuglcna 

 (word for word, pretty eye), which he had given to a species 

 of the Flagcllata provided with ocular spots. This interpre- 

 tation had been questioned by all the authors of his time, 

 and especially by Dujardin." At the present day, however, 

 many distinguished French naturalists hold the same opinion 

 as Ehrenberg — viz., that the so-called ocular spots of the 

 Protozoa, are true visual organs. According to M. Pouchet, 

 the ocular spot of Glenodininrn j^ob/jyJiemnx (one of the Pcri- 

 (Hneaj) has without doubt the function of an eye. It always 

 occupies a fixed and definite position in the cell, and it is 

 composed of two parts — a crystalline humour and a choroid. 

 " The crystalline humour is a strongly refractive, hyalin, 

 club-shaped body, rounded at its free end, which is always 

 directed forwards, while the other end is immersed in the 

 mass of pigment which represents the choroid. The latter is 

 clearly determined ; it forms a sort of hemispherical cap, 

 enveloping the posterior extremity of the crystalline humour. 

 In fact, the visual organ of this organism is composed of 



