60 



ANIMAL LIFE 



case of some of the parasitic worms which live in the bod- 

 ies of other animals, the male has an extraordinarily de- 

 graded, simple body, much smaller than that of the female 

 and differing greatly from that of the female in structure. 

 In some cases even as, for example, 

 the worm which causes " gapes " in 

 chickens the male lives parasiti- 

 cally on the female, being attached to 

 the body of the female for its whole 

 lifetime, and drawing its nourish- 

 ment from her blood (Fig. 29). 



A condition known as partheno- 

 genesis is found among certain of 

 the complex animals. Although the 

 species is represented by individu- 

 als of both sexes, the female can 

 produce young from eggs which 

 have not been fertilized. For ex- 

 ample, the queen bee lays both fer- 

 tilized and unfertilized eggs. From 

 the fertilized eggs hatch the work- 

 ers, which are rudimentary females, 

 and other queens, which are fully- 

 developed females ; from the unfer- 

 tilized eggs hatch only males the 

 drones. Many generations of plant 

 lice are produced each year parthe- 

 nogenetically that is, by unferti- 

 lized females. But there is at least one generation each 

 year produced in the normal way from fertilized eggs. 



Some of the complex animals are hermaphroditic that 

 is, a single individual produces both egg cells and sperm 

 cells. The tapeworm and many allied worms show this 

 condition. This is the normal condition for the simplest 

 animals, as we have already learned, but it is an excep- 

 tional condition among the complex animals. 



FIG. 29. The parasitic worm 

 (Syngamus trachealis), which 

 causes the "gapes" in fowls. 

 The male is attached to the 

 female, and lives as a para- 

 site on her. 



