116 THE EYE IN EVOLUTION 



It is interesting that to a certain extent " photoreceptors " may be seen in 

 the vegetable world with an appropriate structural differentiation. Some plant 

 cells, for example, may be raised up and rendered more convex with a lens-like 

 thickening of the cuticle so that they may collect and concentrate the light 

 more easily on the chloroplasts underneath (Haberlandt, 1901) (Fig. 76). 

 This forms a receptor organ comparable to that seen in many animals — a veritable 

 eye. 



The range of photoreceptive mechanisms seen in Invertebrates is 

 wide, and far exceeds in its complexity the degree of vision which has 

 hitherto been functionally demonstrated in many species, but at the 

 same time it is probably legitimate to correlate function with structure 

 to some extent. In the Protozoa we presumably have merely a common 



irritability, from which we may 

 deduce a sentiency without specific 

 characteristics.^ With the appearance 

 of multicellular animals specialization 

 became possible so that some of the 

 cells in the outer layer could acquire 

 a specific response to various types of 

 stimuli. When the receptors thus be- 



FiG. 76.— Protonema of Schlstosteua , , n i-rv. j ■ i ^ -i 



otiMusDACEA. camc structurally dirierentiated, it 



The feeble light available in the may be assumed that a correspond- 



habitat of the plant is concentrated by j^g differentiation in function be- 



the lens-shaped cells upon the chloro- " -i i -n 



plasts underneath. came possible. Four mam groups or 



modalities appeared — mechano-, 

 chemo-, thermo- and radio -receptors ; of these the first was probably 

 the most fundamental, but the last, although originally the least im- 

 portant, in subsequent evolution has far transcended the others by 

 virtue of its greater potentialities in being able to project itself, as it 

 were, into the distance. The development of " distance " receptors and 

 of the projicient senses is late. 



Indeed, it has been suggested that radio -receptors only acquired their 

 attributes as distance-receptors secondarily and that appreciation of light and 

 darkness originated in a ^ahotoreceptor sensitive to a photochemical change in 

 a substance with which it was in contact. The sea anemone appears to possess 

 I)hotoreceptors of this simple kind (von Uexkiill, 1909), and a similar faculty is 

 present in the skin of the ammocoete larva of the lamprey (Parker, 1903-5) and 

 in numerous Amphibians (Nagel, 1896). 



This tendency, of course, is not confined to vision, l^he touch-spots of the 

 skin have been projected in certain Carnivora to the tips of vibrissae so that 

 exploration of the immediate environment is rendered more easy,^ while the 

 glorified mechano -receptoi's of the organs of Corti respond to vibrations from a 

 wide ;: ; ea in space of an amplitude considerably less than the diaineter of the 

 hydrog > atom (von Bekesy and Rosenblith, 1951). Similarly, the heat-spots 



1 p. 36. 



- For a general study, see Fitzgerald (1940). 



Ammocoete larva 

 of lamprey 



