Parasitism and Social Reciprocity. 197 



demand for masculine exertions may decrease, and a shrinkage of mas- 

 culine force ma} r follow. This happens in a great many cases. 



Among the parasitic Lerneans described above there is often a great 

 disparity in the sexes. Since both male and female are parasitic on 

 some third party, the male need not do a thing to support the family, 

 and so he has acquired the most remarkably shiftless habits and reduced 

 anatomy. He generally retains his sense and locomotive organs, while 

 the female loses hers, but the latter obtains a size from ten to one hun- 

 dred times that of the male. And it often even happens that the male be- 

 comes attached as a parasite to his big partner, establishing himself upon 

 her sexual organs. The same state of things occurs among the nematode 

 SpJioerularice. In some species the sexes do not greatly differ. In 

 others the female is three times as large as the male, while in still 

 others the male worms, vastly reduced in size but retaining their form, 

 occupy the female matrix aud remain parasites of their partners in that 

 position ; and, lastl}', there are cases in which the males, parasitic in 

 their females, are reduced in structure so far that little or nothing 

 remains of them except the organs of reproduction. It might bethought 

 that parasitism and its accompanying anatomical degradation could' 

 scarcely go further with safety to the main chance, which in every race 

 is its reproduction and perpetuation, but here .is a worse case. 



The rat entertains a nematode parasite which sports the name Trichoso- 

 mum Ci-ttsxicfinda. The female is 2. 5 millimetres long and the male one- 

 fifteenth as much. The male lives in the uterus of the female, and has no 

 complete digestive apparatus of its own but has to be fed by the female. 

 Sometimes live of these males are found in one female.* There are a 

 good many crustaceans and insects of which the females alone are para- 

 sitical, while the males remain free and take care of themselves. The 

 two sexes, therefore, differ greatly in form, the female being hatched as 

 a parasite, and remaining during her whole life close to the spot in 

 which she was born, while the male in the case of some of the insects 

 assumes wings and has a gay and varied existence. 



Not less remarkable than the foregoing is the history of the Diplo- 

 :.<><ni. This was at first thought to be a worm with two heads and two 

 tails. But it was subsequently found to be two worms soldered together. 

 They are said to be hermaphrodite from the time they are hatched from 

 tlie erg till toward maturity. In their youthful days they are furnished 

 with cilia for producing locomotion, and with a pair of eyes, and also a 

 small ventral sucker and a dorsal teat or papilla. At maturity they 

 settle in pairs on a host, preferring the gills of a fresh-water fish, each 

 pair crossed like the letter X and t':rst.cncd together by the teat and sucker, 

 in which position they become soldered together and remain so for life. 



* Van Beueden, 251, on the Authority of Lc-uchart, 



