216 NOTES AND MEMORANDA. 



cess of the iris, whicL. covers the upper half of the pupil but which 

 is drawn up at night. This process gradually returns to its position 

 if an artificial light be shown. I have, however, also seen that the 

 pupil of the gurnard {Trigla cuculus) which is almost diamond- shaped 

 by day, enlarges somewhat and becomes circular at night. 



It is difficult to correlate this power of contracting the iris among 

 fishes with any special feature in the powers of vision or even with 

 nocturnal habits. The skate and dog-fish in the tanks move very 

 little by day and seem to find find their food entirely by touch and 

 smell, while the angels remain completely buried until night. 



On the other hand, in such typically nocturnal fish as conger and 

 soles there is no such mechanism of accommodation. It may be 

 mentioned that the turbot sees very well by day and will rise to 

 catch food falling in the water. '-'' 



The eyes of the Elasmobranchs glow in the light of the lantern like 

 a cat's eye, but the eyes of the other fishes in the tanks do not. 



The iris of Cephalopods [Eledone and 8epwla) contracts for light 

 like that of a warm-blooded animal, leaving a slit-like pupil. The 

 size of the pupil in Eledone varies also with the emotions of the 

 animal. I found that it contracted more for green light than for 

 yellow and least of all for red. In the tanks the Sepiolas sit on the 

 ground with their eyes closed by the lower lids throughout the day. 



Modes in which Fish are affected by Artificial Light. — If the fish in 

 the tanks are looked at by night with a lantern several somewhat 

 interesting phenomena may be seen. Fish are differently affected 

 according as they are day or night feeders. Soles and rockling 

 stop swimming if a light is shown, and the former bury themselves 

 almost at once. Bass, pollock, mullet, and bream generally get 

 quickly away at first, but if they can be induced to look steadily at 

 the light with both eyes they gradually sink to the bottom of the 

 tank, and on touching the bottom commonly swim away. Some- 

 times the fish will lie close to the glass, tui-ning one eye only to the 

 light ; in this case the animal never lies horizoutall}^, but always 

 with that side of the head depressed which is turned towards the 

 light. In this connection I may mention that I have seen a whiting 

 which had lost one eye by disease which always swam with the 

 blind side higher than the normal one. In the case of mullet effects 

 apparently of a mesmeric character sometimes occur, for a mullet 

 which has sunk to the bottom as described will sometimes lie there 

 quite still for a considerable time. At other times it will slowly 

 rise in the water until it floats with its dorsal fin out of the water, 

 as though paralysed. I once saw one which remained in this odd 

 position for some minutes after the light had been turned off it. I 



