102 G. WITENBERG 



part of its body in which they are concentrated^^. When the 

 earthworm dies, the larvae are liberated by disintegration of its 

 body and use its decaying tissues as nutrients. They maturate 

 and multiply on the spot as long as the remains of the host last 

 and eventually their larvae disperse to start the new life-cycle in 

 another earthworm^^. 



Some encysted larvae (or their unencysted homologues living 

 free in the tissues) can re-establish themselves if they are swal- 

 lowed by an animal which is unsuitable as a final host. Thus, 

 many insectivorous animals harbour a great number of re- 

 established larvae which were originally encysted in insects; for 

 instance, larvae of the dog nematode, Spirocerca lupi, larvae of 

 the cat cestodes of the genus Diplopylidimu etc. Some carnivores 

 may in this way acquire a great number of tetrathyridia (= 

 larvae of the cestode genus Mesocestoides) which originally 

 developed in mice which served as prey. Big predator fish may 

 accumulate a great number of plerocercoids (= larvae of the 

 cestode genus Diphyllobothrium) which originally developed in 

 small fish. All these re-established larvae remain in the new host 

 in their original undeveloped resting stage. 



RESTING OF NEMATODE LARVAE IN THE UNBORN FOETUS 



An organism may, under certain circumstances, acquire its 

 parasites in utero. Numerous examples of prenatal infection are 

 known in various animal groups. (For example, infection of 

 tick's brood with piroplasms, human prenatal infection with 

 toxoplasma, etc.). Several helminths may infect their hosts in 

 this way. Two species of nematodes, namely, Neoascaris vitu- 

 loriim in calves and Toxocara cam's in the dog, are the best known 

 examples, for they show special predilection for the intrauterine 

 route of infection!^. The eggs of these parasites containing 

 larvae in the infective stage must be swallowed by the host; the 

 larvae migrate from the intestine to the venous system and reach 

 the lungs. A few burst into the alveoli of the lung and from there, 

 via bronchi and trachea, they reach the pharynx where they are 



