CHAPTER VI 

 THE MORPHOLOGY OF INVERTEBRATE EYES 



RICHARD HESSE (1868-1944) (Fig. 75), one of the greatest of German 

 zoologists, probably contributed more towards the study of the sense organs, 

 particularly the visual organs, of the lower animals than any other single indivi- 

 dual. The greater part of his life was devoted to this subject. Professor of 

 Zoology at Tiibingen in 1901, he became Professor at the Agricultural School 

 in Berlin in 1909, occupied the Chair of Zoology at Bonn in 1914, and was 

 Professor of Zoology and Director of the Zoological Institute at the University 

 of Berlin from 1926 to 1935. His systematic study of the light-sensitive organs 

 of Invertebrates was lai-gely conducted between 1896 and 1908 and included 

 an immense range of types varying from the relatively simple eyes of worms 

 to the highly developed visual organs of Molluscs and Arthropods, an interest 

 which he maintained throughout his long and' fruitful life. As will be seen in 

 the following pages, his studies form the basis of our understanding of the 

 astonishing variation in the morjDhology of the eyes of the Invertebrates. It is 

 surprising how little systematic work has been done on this fascinating subject 

 since his day. 



THE GENESIS OF THE EYE 



It is evident from the subject-matter of the previous chapters that 

 the eye has evolved from remote and lowly origins, far removed in form 

 and in function from the highly specialized mechanism we find in Man. 

 In the most primitive miicellular organism, as we have already seen in 

 the case of the amoeba,^ there is a diffuse reaction to light whereby the 

 entire cell, and particularly its superficial layers, in the absence of any 

 apparent specialization of structure responds by a simple alteration of 

 the sol-gel reversibility of the relatively fluid protoplasm. In its 

 earliest form this would seem to be an imdifferentiated response 

 common to all stimuli (photic, tactile, thermal, chemical) (Pantin, 

 1924-26 ; Folger, 1926-27 ; Mast, 1926-32). A reaction of this simple 

 nature to light is t\^3ical of the Rhizopods, such as the amoeba, 

 but among multicellular organisms we would expect it to be localized 

 preferentially in the cells on the surface which are exposed to the 

 stimuli of the external world ; it thus evolves into a general dermal 

 jihotosensitivity. This may occur sometimes in the absence of known 

 photoreceptors, sometimes in association with them, and it is interesting 

 that even when these are present, the more primitive and less specialized 

 mechanism may dominate the behaviour of the animal in its reactions 

 to light more effectively than the conspicuous receptor organs. 



1 p. 3.-,. 



