FISHES 



lacking. Unlike those of the lamprey, the optic nerve fibres have 

 become myelinated (Bruesch and Arey, 1942). At the chiasma there 

 is a complete crossing of the nerve fibres, frequently in the form of 

 interlacing bundles (Figs. 318 and 319) (Verrier, 1930). 



THE EXTEA-ocTJLAR STRUCTURES. The jDresencc of mobile eyelids, 

 both upper and lower, sometimes with an additional fold constituting 



289 



Fig. 317. — The Area Centralis {ac) of the Dogfish, Mustelvs. 



Xote the increase in length and concentration of the visual cells and 

 the great number of ganglion cells (after Franz). 



a third or nictitating membrane in many selachian species is a curious 

 anomaly in the eyes of a fish (Fig. 299). These structures are supplied 

 with an elaborate musculature blended with the muscles of the spiracle; 

 a superficial layer comprises a retractor palpebrse superioris and a con- 

 strictor spiraculi, and a deep layer consists of a levator palpebrse 

 nictitantis, a depressor palpebrae superioris and a dilator spiraculi, the 



Figs. 318 and 319. — The Chiasma of Selachians (Verrier, 1930). 



Fig. 318. — The dogfish, Squalus. 



Fig. 319.— The skate, Raja. 



different elements being more or less blended. The palpebral muscles 

 are supplied by the seventh nerve, the muscles of the nictitating mem- 

 brane by the maxillo-mandibular division of the trigeminal (Ridewood, 

 1898 ; Harman, 1899-1903). The lids are well developed in the deep- 

 sea sharks of the requin family {Galeorhinus) wherein the outside of the 

 nictitating membrane is clothed with the same type of minute placoid 

 scales as is the outer surface of the lower lid. Occasionally there is 

 merely an immobile circular lid-fold in which case a nictitating membrane 

 alone is present (the bonnet shark, Sphyrna tihuro). The purpose of these 

 elaborate lids is difficult to imagine ; Franz (1905) concluded that they 

 were not used to escape from the dazzling of bright light. 



