FISHES 



311 



by a few large sej^ta (the sword-fish, Xipliias, the eel, Anguilla), but as 

 a general rule the simple ependymal core of the Cyclostomes has 

 developed into a more mature system wherein the oligodendroglial 

 cells are scattered in a nerve which is not sharply fasciculated (Prince, 

 1955). At the chiasma a total decussation of the nerve fibres occurs, 

 sometimes as a simple crossing of two intact nerves, occasionally (as 

 in the herring) one nerve button-holing through the other, or crossing 

 in the form of interlacing bundles (Hannover. 1852 ; Parker, 1904 ; 

 Mayhoff. 1912 ; Verrier. 1930) (Figs. 355 to 357). 



It is interesting that Kasquin (1949) reported re-mj-elination of the optic 

 nerve and the return of vision 4 weeks after section of the optic nerve in Astyanax 

 mexicanus, provided the cut ends of the optic nerve were approximated. 



Figs. .355 to 357. — The Chiasma of Teleosts. 



Fig. 355. — The usual con- 

 figuration : the simple 

 crossing of intact 

 nerves. 



Fig. 356. — The herring : 

 the button-holing of 

 one nerve bv another. 



Fig. 357. — The parrot- 

 fish : the interlacing of 

 bundles. 



THE OCULAR ADXEXA. The eyeball is marked off from the surface 

 of the head by a circumocular sulcus, a shallow depression between 

 the corneal epithelium and the skin running circumferentially around 

 the globe ; tliis represents the conjunctival sac and affords the globe 

 the small liberty of movement it possesses. The outer margin of this 

 sulcus constitutes a poorly developed lid-fold— the only representa- 

 tive of eyelids. Such a rudimentary arrangement is in marked contrast 

 to the relatively well-formed lids in Selachians. ^ In a number 

 of swift -swimming pelagic types, however, particularly the herrings 

 (Clupeidse) and the mackerels (Scombrida?), the eye is partially covered 

 by adipose lids, thui cutaneous folds often enclosing fatty tissue 

 arising from the outer lip of the circumocular sulcus. They are usually 

 vertically disposed, one anteriorly and one posteriorly so that when 

 these fids are well developed the aperture is a narrow vertical elHpse, 

 as in the skip-jack, Pomolohus ; occasionally they are fused so that 

 the globe is covered except for a circular opening opposite the pupil, 

 as in the mullet, Mugil ; rarely the skin-folds fuse completely across 

 the eye (as in the anchovy, Engraidis, and relatives of the herring 

 such as Chanos) (Hein, 1913 ; Walls, 1942). In this last event the 



1 p. 289. 



