ANIMAL PAjRASITL'S AND MESSMATES. 



683 



eggs. The tilaria at last is so entirely atrophied that Prof. Jacobson, 

 after seeing it alive on one of his patients at Copenhagen, wrote to 

 Blainville : " This luedina worm is not reallv a worm ; it is a sheath 

 full of eggs." In fact, all the internal organs disappear, and nothing 

 is found in their place except the eggs and their embryos. 



Fig. 20. Female Chigoe. 



Fig. 21. Young Pilaria of Medina. 

 1. Anterior Extremity ; c, Mouth. 2. Caudal 

 Extremity ; d. Anus. 3. Section of the 

 Body. 



The ichneumons and many other insects that lay their eggs in the 

 living larvae of other species, belong to a class of parasites that be- 

 gin life as dependents, but that become free and self-supporting on 

 arrival at adult age. The (Estnis^ or gadfly of the horse (Fig. 22), is 



Hinder Part. 



Fig. 22. (Estrus of the Horse. A.ntef.ior Pakt. 



thus dependent in its early life. But, instead of making their attacks 

 on those of their own class, the gadflies prefer to install themselves 

 on mammals, and sometimes even on man. The eggs are received 

 into some cavity of the body, nostrils, stomach, or a hole in the skin, 

 where they hatch and where the larvte feed until the adult state is 

 reached, when they escape and afterward live in freedom. 



There is a large class of parasites generally known as worms, char- 

 acterized by the circumstance that during their lives they undergo 

 certain strano^e transformations that can only take place by the pas- 



