CHAPTER XIX 

 NEMATODA. FAMILY I. ASCARIDiE 



The Large Roundworms of the Intestine 



Nematoda (p. 217). The nematodes of this family have the body- 

 relatively thick (Fig. 125). The mouth is commonly provided with 

 three lips which may be prominent or inconspicuous and often bear 

 papiUae. When three lips are present one is dorsal, the other two sub- 

 median, touching on the ventral median line (Fig. 121). The males 

 are somewhat smaller than the females and usually have the caudal 

 extremity curved ventrally in the form of a hook. There may be one 

 spicule or two. The females have two ovaries and they are oviparous. 

 So far as known, development in all which are parasitic in warm-blooded 

 animals is without intermediate host and infection is direct. 



All of the ascarids live as parasites in the intestines of their hosts, 

 though they may be found in other organs and in the body cavities 

 reached by their migrations. They Hve free in the in- 

 testinal contents, obtaining their sustenance by absorp- 

 tion of the partly digested nutriment of their host 

 through their simple alimentary tube. 



Investigations as to Life History. — Investigations 

 by Capt. F. H. Stewart (F. H. Stewart On the Life 

 History of Ascaris lumbricoides, British Medical Jour- 

 nal, 1916, Vol. 2, No. 2896) have brought results of 

 great importance bearing upon the life history of Ascaris ^i^- 121.— Dor- 

 lumbricoides and closely related forms. In these experi- j^ ™toemity' of 

 ments Stewart found that if rats or mice were fed ascarid, showing 

 Ascaris eggs, the eggs hatched in the alimentary tract superior median 

 and the embryos migrated to the liver, spleen, and lungs, lateral lips. 

 During these migrations they passed through cei'tain 

 developmental changes, and many of them finally again reached the 

 alimentary tract by way of the lungs, trachea, and esophagus. Within 

 the alimentary tract they did not continue their development and were 

 soon expelled with the feces, so that rats and mice surviving the pneu- 

 monia commonly caused by the invasion of the lungs became free of 

 the parasites as early as the sixteenth day after infection. 



From these findings Stewart concluded that in infection with Ascaris 

 lumbricoides it is necessary in the life cycle for the eggs to be swallowed 

 by rats or mice, and that the embryos hatching from the eggs undergo 



