NOTES AND MEMORANDA. 215 



it is the posterior end or tlie middle of the body which is affected. 

 I have seen these spots vanish suddenly, but sometimes they remain 

 for several hours. It does not seem that these appearances are of 

 the nature of secondary sexual characters, for they appear on conger 

 of all sizes. These spots are, of course, caused by contraction of 

 the chroraatophores in the skin, but they do not appear to be con- 

 nected with light, for they not only are occasional in their occur- 

 rence but once they appeared on a blind conger also. They do 

 not appear to indicate any special emotion or diseased state, as fre- 

 quently the animals thus affected are seen to feed like the rest. 



Contractility of the Iris in Fishes and Cephalopods — While in warm- 

 blooded animals the size of the pupil is regulated by the accommo- 

 datory mechanism of the iris, this power appears to be wanting 

 amongst Teleostean fishes in general. I have examined the eyes of 

 conger, soles, mullet, wrasse, pollock, &c., and have never seen any 

 alteration in the width of the pupil either by day or night or in 

 twilight, neither do they contract when a strong light is flashed on 

 them by night. On the other hand, all the Elasmobranchs living in 

 the tanks are provided with a means of altering the size of the pupil. 

 In the skate this takes the form of the well-known fern-shaped pro- 

 cess from the upper edge of the iris which by day covers the whole 

 pupil. This structure has often been described, but I have found no 

 nieution of the fact that it is gradually drawn up in twilight and com- 

 pletely so at night, leaving the pupil clear. If a bulPs-eye lantern 

 be turned on to one eye, this process very slowly descends again, and 

 in about fifteen or twenty minutes it will reach down over half the 

 pupil. Probably if the exposure to light were continued it would fall 

 into the position which it occupies by day, but the skate always swam 

 off after about twenty minutes. When the animal turned round, it 

 could be seen that the process of the eye on the dark side had also 

 descended to the same degree as on the light side. 



In the dog-fish, nurse and angel-fish, the pupil is almost completely 

 closed during the day by the iris, the edges of which nearly meet 

 along a slit-shaped opening which extends more or less diagonally 

 from the upper posterior edge to the lower anterior one. This slit 

 gradually opens as twilight comes on and in the night the whole of 

 the pupil is exposed. When the light of the lantern was turned into 

 one eye of a dog-fish or nurse, the iris very slowly contracted until 

 the edges met as by day. When the animal turned round the other 

 pupil was seen to be still open widely as before. 



The turbof^ is the only bony fish in which any great change in 

 size of the pupil was seen. This fish has by day a downward pro- 

 * 1 have since seen the same changes iu the pupil of the brill. 



