236 ADAPTATIONS TO NOCTURNAL ACTIVITY 



As the fish grows up and the head tissues become opaque, the argentea 

 covering the chorioid loses most of its meaning, though in the enucleated 

 eye it can still be seen shining through the transparent sclera. Where 

 it continues over the face of the iris, however, it has been claimed to 

 serve as a mirror, reflecting light (enough?) toward crannies and crevices 

 into which the fish happens to be trying to look. The head-mirror worn 

 by a physician, which he pulls down so that the hole in it is opposite his 

 eye when he wants to peer down our gullets, might have been copied by 

 its inventor, Czermak, from the argentea layer of a fish's iris. Whether 

 useful in this way or not, the iridic argentea naturally adds to the opacity 

 of the iris (Fig. 67, a, p. 159). By reflecting much of the light, the 

 guanin leaves less for the melanin of the rather thin fish iris to absorb. 



Guanin in Retinal Tapeta — One of the cleverest uses of guanin is 

 in the retinal tapetum lucidum seen in a few European freshwater fishes 

 and very recently found by George Moore in one of our native species, 

 the pikeperch Stizostedion vitreum. Known in Abramis brama for about 

 a century, and in Rutilus rutilus and Lucioperca sandra for decades, 

 this type of tapetum has been described by its chief student, Wunder, 

 also for Blicca bjorkna, Pelecus cultratus, Acerina cernua, Lucioperca 

 yolgensis, and (provisionally) for Abramis ballerus and Acerina 

 schratzer. Wunder found all of these fishes in Lake Balaton, in western 

 Hungary. Some of them are the most abundant of the 37 species of 

 fishes in that unusual body of water. The 'Balatonsee' is peculiar in that, 

 while enormous in area, it is everywhere shallow — averaging 6 feet in 

 depth; and its waters are turbid almost to the point of opacity for nearly 

 the whole of the year. These tapetum-bearing fishes are quite definitely 

 adapted to this environment, but were of course pre-adapted (see p. 388) 

 before ever they got into it, for most or all of them occur elsewhere in 

 Europe as well. Moreover, the above assemblage of fishes represents at 

 least two separate productions of the same sort of tapetum, for some of 

 the genera {Abramis, Rutilus, Blicca, Pelecus) are cyprinid, malacopter- 

 ygian, fishes; while /4cenn£7 and Lwciopercc? (the latter a close relative of 

 our Stizostedion) belong to the perch family among the Acanthopterygii. 



The retinal guanin tapetum may be small, or may form a huge hori- 

 zontal oval area which practically fills the fundus. It will suffice to 

 describe it for one of the best-known cases, and figure it for another : 



In the superior two-thirds of the fundus of Abramis brama, a normal 

 amount of fuscin pigment is present in each retinal pigment epithelial 



