LIFE HISTORY OF ASCARIS 275 



Ripe eggs ingested by rats and other rodents hatch in the 

 small intestine. Some of the newly-hatched larvae may be elimi- 

 nated in the fseces, but others penetrate the wall 01 the intestine 

 and go to the liver and lungs 

 and occasionally other organs, 

 including the abdominal cavity. 

 The newly-hatched larvae mea- 

 sure between 0.2 and 0.3 mm. 

 in length, but during their 

 migrations and during their so- 

 journ in the lungs they grow Wl B /\ 



rapidly, reaching a length of 

 from 1 to 1.5 mm. by the tenth 

 day, when ready to leave the . FlG : 109 i ^ )e T elop l m ^ n 1 tal sta K g T of 



Ascaris; a, freshly hatched larva; 6, larva 

 lungS. They may reach the from lung of rat on tenth day after infec- 



liver as early as two days after tion - x 130 \ ( Ada P ted from Stewart.) 

 infection, and the lungs on the 



third day. From the sixth to the tenth days the larvae pass from 

 the blood-vessels into the air sacs and bronchial tubes of the lungs, 

 and thence through the trachea to the mouth. Some larvae 

 escape from the mouth, but the majority are swallowed and pass 

 out with the faeces apparently dead, the host becoming free from 

 parasites in about two weeks. It is not probable that, as at first 

 suggested by Stewart, the rat normally acts as an intermediate 

 host, but rather the development of Ascaris to the lung stage and 

 failure to complete the development in the intestine may be con- 

 sidered as a case of imperfect adaptability of the worm to this 

 host. It was subsequently shown by Stewart himself and also by 

 Ransom and Foster that a similar development in the lungs takes 

 place in pigs and other animals as well as in rats, and it is rea- 

 sonable to suppose that a direct development in a single host 

 normally takes place. However, this has been shown to be uhe 

 case only in a few instances, and in cases where a massive infec- 

 tion would be expected from feeding enormous numbers of eggs to 

 susceptible animals only a small number of Ascaris developed, 

 and in some instances none at all. 



Both in the hog and in man, susceptibility to Ascaris infection 

 decreases with age. The higher rate of infection in children is 

 probably, however, due to careless habits as well as to greater sus- 

 ceptibility. 



