158 



ADAPTATIONS TO ARHYTHMIC ACTIVITY 



excised eye contracts somewhat if the retina is stimulated. In the higher 

 vertebrates, at least in adults, reflexes alone are of importance although 

 some direct response is known to occur even in two or three mammals 

 and man; and emotional changes can now affect the pupil, though 

 whether this is incidental or not, useful or not, is difficult to say. In the 

 mammals, 'consensual' reflexes from one eye to the other appear: the 

 movement of both pupils when only one is stimulated (known only in 

 the rays and the pigeon, outside of the mammals) ; and the neurological 

 tie-ups of the pupil to accommodation and convergence become rigid. 



Fig. 65 — Pupillary opercula in fishes (o- operculum ) . 



a, eye of Raja clavata. x 2. After Franz, b, illuminated pupil of R. clavata. After Franz, 

 c, eye of a flatfish, Scophthalmus rhombus, as seen from above. x3. From Franz, after 

 Grynfeltt and Demelle. d, upper part of head of stargazer, Uranoscopus scaber. x 4. 

 Redrawn from Hein. /- lower 'lid'; s- limit of sulcus under lower 'lid', e, f, g, stages in 

 the expansion of the operculum of a loricariid catfish, Plecostomus. Redrawn from Roth. 



Comparative Survey of the Two Methods — In the lampreys there 

 are no iris muscles and most observers agree that the pupil is static. The 

 lens touches the cornea and blocks the pupil, and the mechanism 

 of accommodation (Chapter 10, section A) is such that this relation- 

 ship is never changed. There are no photomechanical changes in lam- 

 preys; but their eyes as a whole are built for diurnality. When lampreys 

 do swim at night, as when going upstream to breeding grounds, they are 

 in all probability depending upon senses other than vision (like diurnal 

 birds migrating at night) . 



