734 



THE EYE IN EVOLUTION 



or vestigial. Thus in the flukes (Trematodes) and in the round- or thread- 

 worms (Nematodes) the eyes may be present in the freely-swimming larval 

 stage but in the parasitic adult sense organs are limited to papillse on the 

 lips. In tape-worms (Cestodes) sense organs are lacking. Similarly eyes and 

 other sense organs are not found in parasitic Crustaceans such as Sacculina, 

 an organism parasitic on the abdomen of crabs. A similar example among 

 Insects is provided by Stylops ; the winged male has many ocelli but the 

 minute female which is parasitic within bugs and bees is eyeless. 



Fig. 881. — The Hag-fish, Mr.xiyE (after Dean). 



Among the Cyclostomes, the hag-fishes have rudimentary eyes which 

 give no response to light. These small eel-like creatures live partly in the 

 mud at the sea bottom and are partly voraciously parasitic within larger 

 fishes. The glutinous hag, Myxirie, with a wide distribution in the oceans, 

 approaches more nearly than any other Vertebrate the condition of an 

 internal parasite (Fig. 881) ; in other species such as the slime-hag, Epta- 

 tretus in the Southern Hemisphere and BdeUostorna, found in South African 

 and Pacific waters, the eyes are not so degenerate (see Henckel, 1944). The 



eyes of the ecto-parasitic lamprey, on the other 



hand, are well-formed.^ 



In Myxine glutinosa, the eyeball, about 0-5 mm. in 

 diameter, is merely a simple vesicle lying in fat buried 

 beneath the skin, almost entirely filled with a poorly 

 differentiated retina doubled over upon itself (Fig. 882). 

 Extra-ocvilar muscles, cornea, iris and ciliary body are 

 unrecognizable, the sclera and choroid are undifferen- 

 tiated, the lens is lacking, and there is no pigment either 

 in the uvea or retina. The retina retains the form of the 

 cavity of the optic vesicle, visual cells are not recogniz- 

 able as such, layering of the retinal elements is crude and 

 the optic nerve is vestigial (Kupffer, 1868 ; Kohl, 1892 ; 

 Retzius, 1893 ; Allen, 1905 ; Eigenmann, 1909 ; and 

 others). 



It is noteworthy that although the eyes are function- 

 less, a dermal sensitivity to light exists concentrated 

 particularly in the head and cloacal regions and disappearing when the animal is skinned. 

 There is a long latent period of about 20 sees, before the animal commences to swim and 

 thereafter to burrow. The photochemical reaction is associated with vitamin A j^ and the 

 response is mediated nervously through the spinal cord (Newth and Ross, 1955 ; Steven, 

 1955). 



Fig. 882. — The Eye of the 

 Hag-fish, Myxine gluti- 



AOSA . 



The eye is a simple vesicle 

 almost entirely filled with 

 poorly differentiated retina 

 doubled over itself (after 

 Diicker). 



Endo -parasitic Fishes are rare, and the parasitic habit is not found among higher 

 Vertebrates, The eel, Simenchelys parasitica, an inhabitant of deep seas and parasitic 



263. 



