ELASMOBRANCH RETINA; CHONDROSTEAN EYE 569 



The optic nerve has various cross-sectional shapes and septal patterns 

 in the various species. In some, an axial core of ependymal or glial cell- 

 bodies persists in the adult, reminiscent of the situation in the lampreys 

 (p. 560; see Fig. 162c, p. 559). 



(B) Chondrosteans 



See also pages: 235-6 argentea 



135-7 Fig. 60, taxonomy 238-42 Fig. 96, tapetum lucidum 



150 photomechanical changes 264, 272-3 accommodation 



160, 220-2 pupil 292 binocular 



vision 



174 eye size and shape, optics 381 streamlining 



187 lack of area centralis 415-6 sclera 



200-2 oil-droplets 519-20 color vision 



The living Chondrostei include the twenty-odd species of sturgeons 

 (and shovel-nosed sturgeons) and the two genera of spoonbills, Poly- 

 odon and. Psephurus. Most sturgeons are marine and anadromus; but 

 a few (and the spoonbills) are confined to fresh water. All are bottom- 

 feeders, with scotopically-adapted eyes. The eyes of the spoonbills have 

 had practically no attention, and the ensuing statements apply solely 

 to the sturgeons. 



The Eye as a Whole — The firm margin of the orbit forms a broad 

 horizontal ellipse, and is supported dorsally and ventrally, in the com- 

 mon European sturgeon (Acipenser sturio) and some others (but not 

 A. nasus) by a pair of crescentic bones embedded in the conjunctiva. 

 These have no phylogenetic connection with the scleral ossicles of either 

 teleosts or sauropsidans. There are two venous sinuses in the orbit in 

 most species, but the really important 'packing' around the eyeball con- 

 sists of connective tissue. Concerning the extra-ocular muscles, no peculi- 

 arities are on record. The eyeball and cornea are slightly oblong hor- 

 izontally, but not as much so as in most sharks. 



The sclera is again cartilaginous, as in the elasmobranchs (and indeed 

 all vertebrates excepting the cyclostomes, some teleosts and urodeles, the 

 snakes, and the non-monotreme mammals). Obeying the 'rule' that 

 where a relatively small eye lies in a large body its sclera is dispropor- 

 tionately thick (see p. 415), the scleral cartilage in the largest stur- 

 geons is monstrously thickened in close imitation of the largest sharks 

 (Selache, Rhineodon) and the whales (Fig. 167; cj. Fig. 141a, p. 413). 

 It is of reasonable thickness, however, in such small-bodied, relatively 

 large-eyed species as the American rock sturgeon, A. fuhescens. The 



