PHYLUM NEMATHELMINTHES 131 



are thus brought forth viviparously. Parthenogenesis occurs 

 in some instances, as mentioned below. 



Life Cycles. — A number of extremely interesting and important 

 facts are connected with the developmental cycle of the parasitic 

 nematodes. Some species are parasitic throughout their entire 

 life (Trichinella and blood Filariae), while many are free-living 

 for at least part of their existence (hookworms as larvae, Mermis 

 as adults). Still others are facultative parasites living either 

 free or parasitic as opportunities are offered. Of this last condi- 

 tion, Rhabditis hufonis serves as an excellent example. The 

 young of this species live in mud where they undergo sexual 

 reproduction, both males and females being found. Larvae pro- 

 duced by these sexual individuals may continue to produce in the 

 same manner as the parents, or in case they find their way into 

 the lungs of a frog they become established as parasites and 

 develop only parthenogenetically. The parthenogenetic eggs 

 pass out with the feces and in the mud again pass through the 

 dioecious phase of the life cycle as free individuals. 



Ascaris lumbricoides is a large worm, about 8 inches long, 

 which lives in the intestines of man and the pig. Though the 

 Ascaris in these two hosts look exactly alike, experiments so far 

 indicate that larvae hatched from a worm living in a pig fail to 

 develop normally in man, and vice versa. Until very recently 

 it has been assumed that infestation by this species is by direct 

 introduction of the young into the digestive tract of a new 

 host individual where it becomes established immediately. 

 Recent investigations have demonstrated that the larvae of these 

 ascarids, when introduced into the digestive tract of a new host 

 individual, undergo extensive migration through the organs and 

 tissues of the body, reaching the final position only after having 

 passed by way of the circulatory system into the lungs. Heavy 

 infestations by these larvae cause serious pulmonary disorders, 

 as the larvae penetrate the lung tissues and travel by way of the 

 respiratory passages into the mouth and down the digestive 

 tract to the intestine, where they reach maturity. 



After entering the host the female requires only about 2 months 

 to reach maturity. Some idea of the danger of this species as a 

 human parasite may be gained from the fact that a single female 

 has been found to give off 200,000 eggs daily with a total produc- 

 tion of 27,000,000 eggs by a single worm. The eggs are very 

 resistant, for the heavy shells prevent even strong chemicals 



