NEMATODE PARASITES 



199 



An important duty of public health authorities consists in the system- 

 atic inspection of pork and ham, especially when imported from abroad, 

 to make sure that it does not contain encysted Trichinae. 



ASCARIS 



A species of Ascaris (A. lumbricoides), resembling that already de- 

 scribed in detail but rather smaller in size (cj 15-25 cm., 520-40 cm.), 

 is a common parasite of man all over the world. Normally it inhabits 

 the small intestine though it may wander into other parts of the body. 

 Its presence in the alimentary canal may be determined by the eggs 



B 



Fig. 90. 



Eggs of parasitic worms from the intestine of Man. They are drawn as resting on a slide ruled 

 in squares with lines lo n (i.e. -oi mm.) apart. A, Trichocephalus (the commonest egg of the four) ; 

 B, Ascaris; C, Ancylostoma ; T>, Schistosoma japonicum. 



in the faeces (Fig. 90, B). These eggs are ellipsoidal and are enclosed 

 in a thick envelope with knobbed surface measuring about 60 /x in length. 

 They do not normally show any signs of developing until they reach the 

 exterior where in moist earth the eggs segment and give rise to embryos. 

 If swallowed by a human being these hatch out in his alimentary canal 

 and in about five weeks are sexually mature and producing eggs. 



Trichocephalus 



Trichocephalus (Fig. 91) is a very common and as a rule harmless 

 inhabitant of the alimentary canal of man, where it is found most fre- 

 quently in the caecum. It measures about 40-45 mm. in length in the 



