202 



THE EYE IN EVOLUTION 



squid, Loligo, and the octopus, the two eyes are large and prominent 

 (Figs. 192-3). They are situated conspicuously on either side of the 

 head behind the main body of tentacles, protected in part by the 

 cartilage surrounding the brain and in part by cartilages in their own 



Fig. 192. — Octopus vulgaris (J. Z. Young). 



walls, and provided with rudimentary lids and a set of 4 extra-ocular 

 muscles which confer a wide range of movement on the globe (Hesse, 

 1908 ; Tompsett, 1939) (Figs. 113 and 114). The complex structure of 

 these organs has already been described, ^ and although they rival the 

 eyes of Vertebrates in their morphology, they 

 are simple in type, derived from the epithelium. 

 The close resemblance of the eyes of these 

 molluscs to the cerebral " camera " eyes of 

 Vertebrates is a striking examjDle of convergent 

 evolution whereby Nature achieves comparable 

 results by travelling along entirely different 

 routes. The nervous connections are promi- 

 nent ; in the posterior wall of each eye is a 

 large optic ganglion from which the thick optic 

 lobes lead directly to the closely associated 

 cerebral ganglion ^ (Fig. 698). There is a well- 

 developed olfactory sac behind each eye as well 

 as two statocysts and organs of general sensa- 

 tion, but it would seem that vision plays a 

 dominant part in the behaviour of the animal.^ 



The Common 

 Loligo vul- 



the two large 

 f^yes, one on 

 the head 

 in). 



143. 



575. 



2 p. 5: 



