RUDIMENTARY EYES 733 



Degenerate subterranean lizards (Amphisbsenidge), which are without Hmbs and 

 almost without scales — such as the worm-like Atyiphisbaina punctata of Cviba or the 

 similarly legless Bhineura floriclana abundant in parts of Florida — have eyes equally 

 minute buried beneath ojaaque skin, rarely consisting of more than a capsule of 

 connective tissue enclosing an optic cup and a cellular lens without fibre-formation ; 

 extra-ocular muscles and iris are lacking (Payne, 1906 ; Eigenmann, 1909). It is 

 interesting that both in these snakes and in lizards, Harder's gland is many times 

 larger than the eye. 



Among Mammals a similar degeneration of the eye is seen in a small 

 group of animals with btu'rowing habits which have led to a life of permanent 

 darkness. These fossorial animals have little vision but an exquisitely 

 developed sense of smell on which, indeed, most of them depend for their 

 living ; the eyes are minute in size but relatively well differentiated, 

 almost although not completely covered by skin to which they are adherent. 

 In the common European mole only a minute pore, 0-1 mm. in diameter, is 

 left in the skin through which little but the merest perception of light can be 

 possible. In the blind mole of Southern Europe, Talpa cceca, this aperture 

 is said to be usually lacking (Weber. 1904 ; Kazzander, 1921). In addition 

 to the European moles, this group includes other Insectivores — the South 

 African and Asian golden mole, Chrysochloris (Sweet, 1909), the American 

 water-mole, Scalops aquaticus (Slonaker, 1902) — as well as the marsupial 

 mole, Notoryctes typhJoi^s (Sweet, 1909) and the rodent '' moles " such 

 as Spalax, and EUobius which belong to the hamster branch of the mouse 

 family. 



The eye of the mole, Talpa, may be taken as typical, and appears as if it had 

 ceased to progress from an early stage of embryological development (Lee, 1870 ; 

 Ciaccio, 1884 ; von Hess, 1889 ; Kohl, 1892-95 ; C. Ritter, 1899 ; Henderson, 1952) 

 (Figs. 878-9). The corneal epithelium may consist of a single layer of cells, the iris is 

 sinall but j^resent, and the choroid, unlike the mammalian but like earlier vertebrate 

 types, has a single layer of vessels ; the lens is embryonic and cellular, while the central 

 artery of the retina retains a hyaloid form and grows into the vitreous. In the retina, 

 rods and cones are distinguishable and intermingled with them are cells of an inter- 

 mediate type, but the normal layering of the mammalian retina is evident (Fig. 880). 

 The non-neural parts of the eye are therefore particularly retarded, and it is interesting 

 that Tuscjues (1954-55) found that their relatively normal development could be 

 stimulated by large doses of thyroxine : the globe increased in size, the lids separated, 

 the lens developed with the jsroduction of fibres and the entire organ began to take 

 on the appearance of the eye of sighted animals. 



THE PARASITIC HABIT 



In most internal parasites the eyes are rudimentary or absent for the 

 inside of an animal is as lightless an environment as any ; moreover, the 

 sedentary life associated with parasitism can proceed in the absence of other 

 activities so that, in addition to the recession of the visual organs, those of 

 locomotion and often of digestion are reduced. 



In the large number of endo-parasitic Invertebrates, eyes are lacking 



