228 THE SENSE-ORGA.NS AND PERCEPTION OF PISHES. 



absence of a contractile iris amongst nocturnal or diurnal fishes. 

 In this Journal (N. S., i, 2, p. 215) I have given some account 

 of my experience in this matter, which it may be convenient to repeat 

 here, with the addition of other facts since noticed. 



In the great majority of fishes observed, the shape and size of 

 the pupil do not alter materially for light. Of the exceptions in 

 which such a mechanism is found, some are nocturnal, as the skate 

 and rough dog-fish, while others are diurnal animals, as the turbot. 

 All of the Elasmobranchs which came under my notice are provided 

 with a contractile iris, but the mode of contraction and the form of 

 the pupil differ greatly among them. The eye of the torpedo 

 {T. marmorata) presents the simplest form of this mechanism. In 

 it the pupil is circular by night, but by day the lower limb of the 

 iris rises up so as to close the pupil almost completely, leaving a 

 horizontal slit at the upper part of the eye {v. fig. 8) . In the 

 rough dog-fish, the angel-fish, and the nursehound the pupil is also 

 closed by day, but in it the edges of the iris meet to form an oblique 

 slit passing across from the upper posterior margin of the iris to the 

 lower anterior one. The arrangement in the skate is altogether 

 peculiar, and seems to have no relation to either of these types of 

 mechanism. In it the pupil is covered in daylight by a process of 

 the upper limb of the iris, which falls over it, forming the well- 

 known fern-shaped structure {v. figs. 7a and 7b). This peculiar 

 irideal fold seems to consist of a constant number (eleven) of pro- 

 cesses. By night this fold is completely drawn up, leaving the 

 pupil clear. As described in the place referred to, the pupils of 

 the dogfish and skate contract at night when the light of a lantern 

 is turned on the eye, but this contraction is not sudden, as it is in 

 terrestrial animals and Cephalopods, but, on the contrary, takes a 

 long time to be completed. In the skate the process of the iris did 

 not completely descend when the eye had been exposed to light for 

 about twenty minutes, but the pupil of the dog-fish was almost 

 entirely closed in that time. Illumination of one eye only in the 

 skate causes the irideal fold of both sides to descend simultaneously, 

 but the pupil of the dog-fish remained open on the dark side when 

 the folds of the iris had nearly met in the illuminated eye. 



The eye of the sterlet (Acipenser) , a night-feeder, also has a con- 

 tractile iris, which is arranged as a circle which is incomplete at the 

 upper edge {v. fig. 10). 



Among diurnal fishes, the turbot and brill, together with the 

 weever, all have a semicircular flap from the upper edge of the iris 

 which partially covers the pupil by day, but is almost entirely 

 retracted at night, slowly returning under the light of a lantern. 

 In the Brighton Aquarium I saw a turbot in which this flap of the 



