678 



THE EYE IN EVOLUTION 



Onychophore, 

 Peripatus 



Centi- Milli- 

 pede pede 

 (Myriapods) 



Daphnia 



Lobster 



der 



performance of Arthropods. Among the lower types the tactile sense 

 takes pride of place in biological utility ; in Insects vision is dominant 

 with the sense of smell (centred in the antennae) a good second. 



The ONYCHOPHORA are provided with eyes which merely differen- 

 tiate the presence or absence of light from which the creature 

 persistently flees. A crude image-formation is possible among the 

 MYRIAPODS ; although Lithohius is trainable to the extent that it can 

 master the single turn of a simple T-maze, it does so by its tactile 

 sense on the basis of the texture of the walls (Scharmer, 1935). The 

 visual sense of the smaller crustaceans is almost certainly similarly 

 crude, but light perception at any rate, with phototactic responses 

 while swimming is well developed. In the Cladocera, particularly the 

 water-flea, Daphnia, it has been established by a large number of 

 observers that the phototactic response varies with the wave-length of 

 light so that a differential sensitivity would appear to exist, particu- 

 larly affecting red and bhie.^ Moreover, an elementary degree of 

 training is possible even in these minute creatures since the positive 

 taxis of Daphnia to a source of light through a narrow tube can be 

 rendered less clumsy with experience (Blees, 1918) ; but any such 

 feat as the negotiation of the single turn of a T-maze seems to be beyond 

 the capacity of the small Crustaceans {Daphnia and Simocephalus, 

 Agar, 1927). These creatures thus seem to be inferior to earthworms 

 in this respect. 2 Some directional sense to light stimuli is probable, 

 and Exner (1891) suggested that the Copepod, Copilia, made the most 

 effective use of its simple ocular apparatus, by scanning movements of 

 the stalk-like eye controlled by its system of muscles (Fig. 139). 



Not much more is known about the visual functions of the higher 

 Crustaceans, although the anatomical elaboration of their compound 

 eyes with their complex nervous connections would indicate visual 

 potentialities of considerable proficiency. In the lobster, for example, 

 optomotor reactions are readily elicited when the animal is confronted 

 with a black-and-white striped rotating drum ^ ; moreover, reactions 

 differ depending on the colour of the stripe, suggesting the presence of a 

 colour sense or, at any rate, a differential reflex action to different 

 wave-lengths of light. ^ Many of these animals, however, are essentially 

 nocturnal or frequent ocean depths where the paucity or absence of 

 light must preclude acute vision. It is probable, indeed, that as 

 determinants of behaviour the eyes are of secondary importance to the 



1 V. Frisch and Kupelwieser (1913), Ewald (1914), Koehler (1924), Eckert (1935), 

 Heberdey (1936), Heberdey and Kupka (1942), Hmith and Baylor (1953). It is to be 

 remembered that these differential responses may be served by different mechanisms — 

 tlie dermatoptic and the ocular. 



2 p. 573. 



^ Homarus — v. Buddenbrock et al. (1952). 



* Schlieper (1926-27), Kastner (1949) in the crab, Curcinus, the shrimp, Crangon, 

 a,' the prawn, Leander. 



