HERMAPHRODITISM 169 



to settle down on an adult bonellia they become males ; 

 but if the larvae have no such opportunity a long period 

 without further development intervenes, and later the 

 larvae become, for the most part, females, a few become 

 males, and a few hermaphrodites. Bonellia appears 

 therefore to be a protandric hermaphrodite, — like many 

 plants. Somewhat similar relations are known for Hydra 

 viridis, as shown by Nussbaum, and by Whitney. 



In cases where a sexual generation alternates with 

 a hermaphroditic generation, the problem of the two 



Fig. 86. — Rhabditis nigrivenosa, male (left) and female (right). (After 



Leunis.) 



sexes reappears. There is but one case in animals 

 that has been adequately worked out. A nematode 

 worm, Rhabditis nigrovenosa, lives as a parasite in 

 the lungs of frogs. It is an hermaphrodite. Its 

 eggs give rise to another generation that lives in mud 

 and slime. In this generation two kinds of individuals 

 are present — true males and females (Fig. 86) . The 

 females produce eggs, that are fertilized, and develop 



