FI8HES 



327 



accommodating maximally the eyes are focused for aerial vision, and 

 to adapt the vision to the bright sunlight on land, the inferior part of 

 the retina is populated only by cones, while rods become increasingly 

 more numerous in its upper half. 



The hammerhead shark, Sphyrna zijgcena, has eyes which are 

 located far laterally at the ends of the " hammers", and show a peculiar 

 adaptation of the extra-ocular muscles (Fig. 388). The elongation of 

 the orbits in the lateral direction would ordinarily necessitate muscles 



Figs. 387 and 388.— The Hammerhead Shark, Sphyrsa zycesa. 



Fig. 387. Fig. 388. 



Fig. 387.— The dissected orbit. 



Fig. 388. — The head, showmg the extraordinarily elongated orbits giving 

 the impression of the heads of two symmetrical hammers on which the eyes 

 are perched (Bland-Sutton's Lectures and Essays ; Heinemann). 



of quite unusual length ; these, however, are no longer proportionately 

 than in any other sjDecies of shark and, instead of being inserted at the 

 apex of the orbit, take origin from a common tendon running parallel 

 with the optic nerve throughout the inner three-quarters of the orbit 

 (Bland-Sutton. 1920) (Fig. 387). The bonnet-shark, Sjjhyrna tiburo, 

 has a head of a somewhat similar configuration, taking the shape of a 

 crescent with the eyes situated on the widest part. 



The most extraordinary stalked eye among Teleosts, however, is 

 seen in the Sti/lophthalmus j^aradoxus, the larva of the deep-sea Idia- 

 canthus (Brauer, 1908 ; Beebe, 1934). The eye is i^erched on an 

 enormously long, freely movable stalk wliich contains the optic nerve 

 and filamentous muscles and is supported by a cartilaginous rod 



Sphyrna tiburo 



