The Senses of Animals. 287 



the iris. The contraction in these cases does not seem to 

 take place rapidly as in land vertebrates, but slowly and 

 gradually. 



Among diurnal fishes belonging to the group of the 

 bony fishes (Teleosteans), the turbot, the brill, and the 

 weever have a semicircular flap from the upper edge of 

 the iris, which partially covers the pupil by day, but is 

 almost wholly retracted at night. 



None of the fishes observed by Mr. Bateson appears to 

 distinguish food (worms) at a greater horizontal distance 

 than about four feet, and for most of them the vertical 

 limit seemed to be about three feet ; but the plaice at the 

 bottom of the tank perceived worms when at the surface of 

 the water, being about five feet above them. Most of them 

 exhibited little power of seeing an object below them. But 

 though the distance of clear vision seems to be so short for 

 small objects in the water, many of these fish (plaice, 

 mullet, bream) notice a man on the other side of the room, 

 distant about fifteen feet from the window of the tank. 

 The sight of some fishes, such as the wrasses (Labridce), is 

 admirably adapted for vision at very close quarters. " I 

 have often seen," says Mr. Bateson, "a large wrasse search 

 the sand for shrimps, turning sideways, and looking with 

 either eye independently, like a chamaeleon. Its vision is 

 so good that it can see a shrimp with certainty when the 

 whole body is buried in grey sand excepting the antennae 

 and antenna-plates. It should be borne in mind that, if 

 the sand be fine, a shrimp will bury itself absolutely, 

 digging with its swimmerets, kicking the sand forwards 

 with its chelss, finally raking the sand over its back, and 

 gently levelling it with its antennee ; but if the least bit 

 be exposed, the wrasses will find it in spite of its protective 

 coloration." 



Although it is probably not functional in any existing 

 form, mention must here be made of the median or pineal 

 eye. On the head of the common slow- worm, or blind- 

 worm, there is a dark patch surrounding a brighter spot. 



