THE EVOLUTION OF LIFE 



345 



performs a useful digestive function. The fauna of caves 



frequently includes many animals which are totally blind. 



Many of these forms have eyes in a 



rudimentary condition. Among 



the blind fishes of which there are 



numerous species in the caves of 



North America there are various 



degrees of degeneracy to be met 



with in the structure of the eyes, 



from those in which the eyes are 



fairly well developed, though func- 



tionless, to those in which they 



have almost entirely disappeared. 



The blind crayfish of the Mammoth 



Cave has lost its eyes, but it still 



preserves the eyestalks. In the inner corner of our eye 



there is a small semilunar fold, a rudiment of the third 



FIG. 234. Cecum and 

 appendix of man. A, ap- 

 pendix; C, cecum; LI, 

 large intestine; SI, small 

 intestine. 



FIG. 235. Rudimentary hind limbs of Python. (After Romanes.) 



eyelid, which in many reptiles and birds is capable of 

 being drawn over the surface of the eye. Some of the 



