REPRODUCTION AND SEX 



539 



females. But these females were once males! For the remarkable 

 fact has been discovered by Gould that in the species called Crepidula 

 plana the newly fixed forms are mostly hermaphrodites, which 

 change quickly into females; but if these young neutrals are brought 

 into the proximity of large individuals they become males! Or if 

 the young fixed individual was a male, it will remain for a con- 

 siderable time a male, instead of hurrying through that phase, 

 provided a big-sized neighbour is present. 



The big-sized individual may be a male or female; the nature of 

 its influence, that slows in its neighbour the passage into the female 



Fig. 8i. 



Two Stages in the Life-history of Sacculina. After Delage. A, the nauplius, 

 with three pairs of appendage (i, 2, 3). B, the cypris or pupa stage, 

 effecting attachment to a crab; a, antennse; l, swimming appendages. 

 The eye is shown dark in each figure. 



state, remains quite obscure. But this case of the Slipper Limpet is 

 enough to show that sex is more plastic than is usually believed. 



We have already referred to the Mediterranean worm, BoneUia, of 

 a beautiful green colour. The female is about the size and shape of 

 a prune, unwrinkled; but a long, narrow contractile proboscis pro- 

 trudes for ten inches or so from the mouth and ends in two ciliated 

 lobes. But the male is a very different creature, and no ordinary 

 observer will discover him at all. He is degenerate and practi- 

 cally microscopic; he lives inside his giant mate! For our present 

 purpose the interesting point is this: the microscopic larvae of 

 BoneUia are free-swimming for awhile, and after that they settle 

 down. If one settles down in the mud on the floor of the sea, it 

 develops into a female. But if one adheres to the proboscis of a 

 female, it develops into a male ! 



Of great interest in this connection are the studies made by 



