030 SENSORY SYSTEM 



therefore,conclusively shows that the direction of the responsive movement 

 is determined, under the prevailing conditions, by the light-condensing 

 capacity of the cells of the upper epidermis. The " lens-action " of 

 these cells is the deciding factor for one of two reasons : either it is 

 actually responsible for that differential illumination of the inner 

 epidermal walls which represents the first link in the chain of processes 

 included under the heading of light-perception ; or else it accentuates a 

 contrast in illumination, already ensured by other structural peculiarities, 

 to such an extent that the resulting stimulus is enormously intensified. 

 In a certain sense, therefore, light-condensing epidermal papillae may 

 be regarded as " optical stimulators." 326 



C. EYE-SPOTS. 327 



A long familiar feature of many Flagellates (e.g. the Euglenaceae) 

 of certain Peridineae, and of the swarm-spores of most Green Algae, is 

 the presence of a so-called eye-spot (or "stigma") ; it is probable that this 

 structure plays a prominent part in connection with the perception of 

 light by the aforesaid organisms. 



The eye-spot is a well defined cell-organ ; it is discoid, lenticular, 

 or rod-shaped, and is rendered very conspicuous by its red or 

 brown coloration. Among Flagellates it consists according to Klebs, 

 Schilling and France of a protoplasmic stroma, in which granules of 

 colouring matter are embedded ; in Volvox and in the swarm-spores of 

 Chlorophyceae, on the other hand, it appears, from Overton's descrip- 

 tion, to be an entirely homogeneous structure. There is little reason to 

 suppose that the colourless inclusions of the eye-spot (granules of 

 starch or paramylum), observed by France, act as light-condensing organs. 

 Sometimes the eye-spot is located in the polioplasm, not infrequently in 

 close relation to a chromatophore (Chrysomonadineae, Cryptogkna, Dra- 

 parnaldia). In other cases it is situated in the ectoplast, or rather repre- 

 sents a local modification of the external plasmatic membrane ; this 

 condition is exemplified by the Euglenaceae, in which group, moreover, 

 the eye-spot abuts immediately against the principal vacuole. In 

 Cladophora laetevirens, according to Strasburger, it represents an exter- 

 nally projecting local thickening of the ectoplast, underlain by a 

 lenticular cavity in the cytoplasm, which is filled with some homo- 

 geneous substance. 



There are various grounds for attributing a light-perceiving function 

 to eye-spots. The location of these organs at the anterior ends of 

 motile cells agrees with this view ; it is, moreover, a significant fact that 

 all Flagellates, Peridineae and swarm-spores which possess eye-spots are 

 also phototactic, though, on the other hand, some Flagellates and Peri- 

 dineae are devoid of eye-spots, and yet respond very actively to photic 



