FISHES 



283 



and rays {Myliobatis) and the basking shark {Selache maxima). It was 

 know to Soemmerring (1818) and has been most fully studied by Franz 

 (1905-34). Structurally it is made up of two elements, highly reflecting 

 cells packed with guanine crystals, and heavily pigmented melanophores. 

 In some species such as the porbeagle shark. Lamna cornubica, the 

 guanophores lie in parallel layers, the interstices between them being 

 occupied by melanophores. In the more typical arrangement, however, 

 the flat silvery guanophores are arranged as a series of plates running 

 in a slanting direction to the choriocapillaris, and over them the 

 chromatophores send pigmented processes. The arrangement as 

 depicted by Franz is seen in Fig. 300. In dim illumination the pig- 





Fig. 300. — The Tapetum Lucidum of the Dogfish, Mr.<Ti:LLs. 



In vertical section, from the dorsal part of the fundus. 



C, choriocapillaris ; PE, pigment epithelium ; PC, pigmented layei- of the 

 choroid ; V, vessels of the choroid ; P, pigmented cells, the processes of 

 which (Pr) migrate over the tapetal plates (T) (after Franz, 1931). 



MyUohatid 



Selache 



mentary processes are retracted and the guanophores appear as a 

 silvery row of plates like the tiles on a roof from which the incident 

 light is reflected back to the retina ; in bright illumination the pig- 

 mented cells send down their migratory processes which cover the 

 guanophores so that all the incident light reaching the choroid is 

 absorbed. 



The ciliary zone of Selachians has some unique features. It 

 is thin and without musculature, occupying a broad belt between the 

 retina and the iris, consisting from without inwards of three layers — a 

 mesodermal layer, the forward continuation of the choroid, a pigmented 

 ectodermal layer, the forward continuation of the pigmented retinal 

 epithelium, and a non-pigmented ectoderm;!. 1 layer, the forward 



Lamna 



