198 



Dynamic Theory. 



FIG. 95. FIG. 96. 



Fig. 95. Two young Diporpa (or two buckles) beginning to attach themselves to- 

 gether. 



O Mouth. Z Papilla. G Sucker. 



Fig. 96. Process complete, the animal being now called Diplozoon (double animal). 

 (After E. Zeller.) 



They each lay eggs which are deposited on the gill of the fish and 

 hatched there. 



These worms are hermaphrodites, but, as is common amongst this 

 class, they are probably not self-fertilizing but require to be paired, each 

 being female to the other. This peculiar sort of marriage takes place 

 with the Lymnece stagnalis, or pond snail. Each individual is both 

 male and female, but the position of the organs is such as to render self- 

 fertilization impossible, so each one becomes a male to a second party 

 and a female to a third at the same time ; from which peculiarity they 

 are sometimes observed joined together in a long string. The separation 

 of the sexes is among the early differentiations by which a loss by one 

 half of a race of one half of its sexual functions and by the other half of 

 the race of the other half of such functions, brings compensation in the 

 establishment of co-operation between individuals, followed by family 

 and social relations. 



Fig. 97. A Trematode worm, Distomum hsematobium , a 

 parasite in the veins of man. Male and female together, the 

 latter contained in the gynecophorus, or wife-holder. 



3-Male. 

 Female, 

 s Sucker. 



Fig. 97 is a trematode which makes its home in human veins' 

 the male the larger in this case, and provided with a sort of 

 pouch for holding his partner. 



There is a parasitic nematode worm called Syngct- 

 mus trachealis, which is developed in the tracheal 

 artery of fowls. These worms are sexual and they 

 are soldered together in pairs a male and a female. 



Van Beneden has a chapter on < ' parasites that 

 are free when old." These he calls parasitic in 

 youth instead of old age. The examples he gives 

 FIG. 97. are such as the Ichneumon fly and other hymenop- 



