CYCLOSTOMEvS 



269 



genus, Ichthyomyzon, the two differ little in size ; in Lmnpefra fluviatilis 

 the difference is marked and in Entosjjhenus it is maximal. In shallow- 

 water forms such as Lampetra fluviatilis and the brook lampreys, the 

 two types are found in apjaroximately equal numbers, while in those 

 which live in deeper waters {Petromyzon marinus, etc.) and presumably 

 demand greater sensitivity to light, the short greatly outnumber the 

 long (Figs. 281 and 282). 



The existence of a duplex mechanism in the retina has not always been 

 accepted and the nature of the cells has long been called in ciuestion. Heinrich 

 Miiller (1857) who first studied the subject in L. fluviatilis, differentiated the 



Fig. 281. — The Visual Cells of the 

 Atlantic Lamprey, PET/to.vrzoy 



MARIXVS. 



Showing the " long "' and the 

 " short " elements ( X 1,000) (Gordon 

 Walls). 



Fig. 282. — The Visual Cells of the 

 New Zealand Lamprey, Geotria 

 australia. 



There are three types of cell in 

 apjDroximately equal numbers, one 

 plump (to the left), one slender (to the 

 right) and an intermediate type 

 (middle) with a eosinophobic ellipsoid 

 ( X 1,000) (Gordon Walls). 



two types of cell, and while initially he called them both cones, he later (1862) 

 suggested that the short elements were rods. Since his time every possible 

 suggestion has been made — that both cell-types are rods (Schultze, 1866-71 ; 

 Franz, 1932) ; that both are cones (Kohl, 1892) ; that the cells are neither rods 

 nor cones but primitive and undifferentiated in type (Plate, 1924 ; Diicker, 

 1924); that the long cells are cones and the short rods (Walls, 1935); or — the view 

 of the majority of workers — that the long cell is a rod and the short a cone 

 (W. Krause, 1868-76 ; Langerhans, 1873-76 ; Greeff, 1900 ; Tretjakoff, 1916 ; 

 R. Krause, 1923). Most of the evidence brought forward in support of these 

 divergent views is morphological in nature — a somewhat dangerous basis for 

 the differentiation of rods and cones. ^ The demonstration by Kiihne (1878) 

 that rhodopsin is present in the retina of the lamprey proves the presence of 

 rods ; the difference in the two types of cell suggests strongly a duplex population; 



' p. 251. 



