80 



THE INFLUENCE OF INANIMATE SURROUNDINGS. 



mole, however, and without exception, both eyes are originally 

 connected with the brain by well-developed optic nerves, and 

 so theoretically efficient. This may indeed be regarded as a 

 perfectly conclusive proof that the blind mole is descended from 

 progenitors that could see; it would seem, too, to prove that 

 the blindness of the fully grown animal is the result not of 

 inheritance, but of the directly injurious effects of darkness on 

 the optic nerve in each individual. 



FIG. 21. a, Pinnotheres Holothuriae of the natural size ; 6, degenerate water -lungs, with 

 the distended portion, c, in which a small Pinnotheres is established. 



To these examples I will add one more which I myself have 

 studied. There is a peculiar family among the crabs, the Pinno- 

 theridte, of which various species live in the branchial cavities 

 of many Mollusca ; some live in Serpulse, and others (fig. 21) 

 which I have found off the Philippine Islands live in the water- 

 lungs, as they have been called, of Holothurians. These are 

 elongated branched tubes in direct communication with the 

 terminal intestine or cloaca, so that parasites can enter them 



