186 REPRODUCTION 



Somewhat similar conditions occur in the free-living 

 Nematoda. The nematodes living in the soil present three 

 different forms of sexual condition : 



{a) separate sexes, 



(b) hermaphroditism, 



(c) parthenogenesis. 



It is believed that the hermaphrodite species have been 

 developed from bi-sexual species by the development of 

 spermatozoa in the female and the suppression of the male 

 sex. In several species males exist, often in considerable 

 numbers, which are completely developed and produce 

 spermatozoa, but yet do not function. The hermaphrodite 

 nematodes are all self-fertilizing, as is the case with Tur- 

 BELLARiA and with flukes and tape- worms. Hermaphroditism 

 in the round-worms is, however, incipient, and not yet fully 

 established, for the number of spermatozoa provided are 

 quite insufficient to fertilize the eggs that are produced. 

 When the spermatozoa are exhausted, the unfertilized eggs 

 go on being laid, but rapidly disintegrate. 



Hermaphroditism can be induced. When the hermit-crab 

 becomes infected by a certain parasite known asPeltogaster, ova 

 make their appearance in the testes of the male crab and its 

 secondary sexual characters put on a female appearance ; and 

 the same is true of another crab, Inachus, when infested by 

 a degenerate crustacean parasite, known as Sacculi7ia. Then 

 there is a certain marine annelid, known as Bonellia, whose 

 larvae after a free-swimming stage become attached to the 

 sea bottom and develop into females or become degenerate 

 males. By releasing the larvae from their support before 

 their sex has become established and forcing them to lead 

 an independent life, it was found possible to produce grades 

 of individuals in which the degree of maleness or femaleness 

 depended upon the duration of the time they had been 

 attached before being released. 



Parthenogenesis 



Parthenogenesis is the development of a true egg without 

 being fertilized by a spermatozoon. It can be produced 

 artificially; but its normal occurrence in nature is very common. 



