540 LIFE : OUTLINES OF GENERAL BIOLOGY 



Geoffrey Smitli and others on crabs that fall victims to parasitic 

 crustaceans distantly related to barnacles. One of the best known is 

 called Saccuhna, which is occasionally found on the sea-shore, 

 protruding hke a kidney-bean on the under side of a crab. The 

 young Saccuhna, after a period of free-swimming, first as a 

 nauplius. then as a cyprid lava, fixes itself to a crab, works its way 

 into the interior, and grows greatly at the expense of the tissues of 

 its host. 



It eventually protrudes on the outside, beneath the shelter of the 

 tail. Let us suppose, for simplicity's sake, that the parasitised crab 

 is a growing male; its constitution soon begins to be profoundly 

 altered. Its tail broadens, approaching that of a female; some addi- 

 tional appendages develop on the tail, which are characteristic of the 

 female, though smaller in size; the composition of the blood seems 



Fig. 82. 



A Crab, Inachus, Parasitised by a Khizocephalan Crustacean, Sacculina (S), 

 which brings about castration. After Geoffrey Smith. 



to be altered; and, most remarkable of all, the male reproductive 

 organ begins to show some eggs. Here we see sex partly changed by 

 a parasite! 



Hut, it may be asked, are these changes of sex confined to the 

 lower animals? The answer is that several cases of sex-reversal 

 in vertebrates have been demonstrated in the last few years. 

 Champy, a Frencli investigator, showed in 1920 that a male newt, 

 which had fertili.scd the eggs of a female, became, as the result of 

 prolonged fasting, indistingui.shablc from a typical female. Its 

 rei)roductive organs were found to be full of immature ova. Very 

 carefully studied is Prof. Crew's ca.se of sex-reversal in a hen. 



Some time earlier, but not quite .so convincingly, Prof. Riddle 

 produced evidence of complete sex-reversal in a ringdove. Both 

 the hen and the male bird suffered from tuberculosis. We have said 

 enough to .show that sex is not necessarily fixed, once and for all. It 

 is pla.stic; it may be reversible! 



