THE SENSE-ORGANS AND PERCEPTION OF FISHES. 229 



iris was hardly developed at all. In speaking of the weever as a 

 diurnal fish it is not intended to assert that it is not active at night. 

 As is well known, it lives buried in the sand by day, but it has eyes 

 which see well in daylight, for it will uncover itself and swim up to 

 food just as a plaice does. As to its habits at night I have no 

 evidence. 



The pupil of the halibut is kidney-shaped, the concavity being 

 upwards. The pupil of the plaice is of the same pattern, and with- 

 out special irideal mechanism. 



The pupil of the gurnard differs from all these in being slightly 

 contracted by day so as to have a diamond shape, while it is circular 

 by night. 



All the fishes attainable were tested with a lantern by night, but 

 in none was any alteration in the size of the pupil observed except 

 in the cases mentioned. It is a somewhat remarkable fact that the 

 reflex contractility of the iris, which is such a general character of 

 land Vertebrata, should be so irregularly developed among fishes. 

 In none of them does the usual sudden contraction for light occur, 

 though it is nevertheless found in the Cephalopods {v. loc. cit.). 

 Moreover, as the matter may have a bearing on the optical aspects 

 of aquatic vision, attention is called to the fact that none of the 

 animals mentioned which are provided with appliances for contract- 

 ing the pupil have the circular aperture which is usual in many ter- 

 restrial animals and also in several of the other fishes which have no 

 special mechanism of this kind. 



The eyes of the three-bearded rockling (uight-feeder) are extra- 

 ordinarily convex, and pi'otrude from the general level of the face so 

 much that the lens can be seen through the cornea on looking down 

 on the animal's head from the dorsal side. The same is true of the 

 eyes of the boar-fish [Cajpros aper), which is a day-feeder; while 

 those of the conger, also a night-feeder, are particularly flat. 



The eyes of fishes are mostly not capable of much rotation, but 

 those of the dory, wrasses, lump-sucker, and pipe-fishes can all be 

 moved, and are used independently of each other (cf . p. 242) . The 

 eyes of the loach arc also moveable, but to a less extent. 



Olfactory Organs. 



In all the fishes examined olfactory organs are present, but 

 their development differs greatly in the different forms, the largest 

 occurring in the eel, the conger, the Raiidce, and the dog-fish, 

 and the smallest in the top-knot [Zeugopterus punctatus) . It might 

 be expected that the olfactory organs of fishes which hunt by scent 



