THE LAMPREY RETINA 



561 



E. tridentatus (8:1; in lamottenii, 1 :1). Lampetra fluviatilis (Fig. 162), 

 though a member of the culminant genus of the petromyzonid line, has 

 its rod and cone outer segments of less unequal length, and even the rod 

 outer segments are slightly tapered — thus, rod and cone are rather less 

 well differentiated than those of Entosphenus, ranking just below Lam- 

 petra. The 1 : 1 ratio of rods to cones in fluviatilis — so close to the ratios 

 in all brook lampreys, regardless of their taxonomic affinities — probably 

 reflects the shallow-water habitat. The rods outnumber the cones most 



a b 



Fig. 163 — Visual cells of lampreys and elasmobranchs. x 1000. 



a, 'cone' types (at least one of them functionally a rod) of New ZealanH lamprey, Geotria 

 australis. 



b, cone and rod of landlocked Atlantic lamprey, Petromyzon marinus. 



c, cone and rod of smooth dogfish, Mustelus cants (redrawn from Schaper). In M. mustelus 

 the cone is less rod-like in form, and in Myliobatis aquila it is fully differentiated; all other 

 elasmobranchs have only rods, like that shown here. 



greatly in the lake and marine lampreys, which, for their life in deeper 

 waters, might be expected to require more rods for greater sensitivity 

 to light. 



On morphological grounds alone, it appears probable that the pet- 

 romyzonid rod has evolved from a cone within the group, with Ichthyo- 

 myzon exhibiting an early stage in the process; and, from taxonomic 

 considerations, it would seem that this rod must then have no connection 

 with any other in the vertebrates (see Plate I). The visual cells of 



