PHYLUM NEMATOIDEA 



185 



Race Preservation. Ascaris is dioecious, that is, the 

 sexes occur in separate individuals. Males are easily dis- 

 tinguished from females by their smaller size, by the coiling 

 of the tail, and by the two little setae which project just in 

 front of the anal opening (Fig. 77). A female has a pair of 

 long tubular ovaries which swing freely in the body-cavity 

 and occupy a large part of its space. The ends of the tubes 

 are enlarged to form a pair of uteri in which the eggs develop 

 to some extent before passing from the body. The males 

 have a single tubular testis which opens through the anus. 



The eggs are fertilized far up in the- ovarian tubules and 

 pass down into the uteri where they have the shell added. 



C D E F G H I J 



FIG. 77. Life History of Ascaris. A, male; B, female; ex, excretory pore; 

 u, female genital aperture; C, zygote; D, zygote without covering: E-H, 

 cleavage stages; I, J, embryos; K, young Ascaris emerging from egg shell. 



A female Ascaris is continually laying eggs, producing as 

 many as 15,000 in a day. A large number of reproductive 

 products is commonly associated with parasitic life and is 

 necessary because the chances of finding a favorable place 

 for development are small. This is, however, the only 

 respect in which Ascaris shows adaptation to parasitism. 

 The eggs pass out of the host and develop into larvae in 

 moist earth (Fig. 77, C-K) but do not hatch unless they get 

 into a man (K). They cannot infect until they have been 

 upon the ground thirty to forty days, and then are intro- 

 duced into the human body on polluted vegetables, in water, 

 or from soiled hands. After gaining entrance the worm 

 hatches, becomes mature, and begins to lay eggs in about a 



