772 PARASITIC DISEASES. 



further changes resulting in the formation of the perfectly 

 mature oxyuris are accomplished within the alimentary canal 

 of the bearer. Here we have cases in which the adult sexual 

 form of the parasite lays eggs in the alimentary canal of its 

 host. Favoured by the warmth and moisture of the alimentary 

 canal, the embryo reaches a certain stage of development while 

 still enclosed within its egg-shell. If these eggs, expelled 

 with the fseces, reach a new host, the embryo is hatched, and 

 develops into a sexual adult. This is the mode of development 

 in Triclioccphalus ajjinis^ and almost certainly in Oxyuris 

 vermicularis of man, Oxyuris curvula of the horse, and Ascaris 

 lumhricoides. 



Thus Dr. Heller foUnd, post mortem, young round-worms, of 

 the species Ascaris lumhricoides, in the intestines of an imbecile. 

 There were eighteen specimens, the largest of which had only 

 acquired the length of about half an inch, whilst the smallest 

 gave a long diameter of only 2-75 millimetres, or let us say 

 roughly the ninth part of an inch. Thus Heller, in part at 

 least, bridged over the gap which had formerly existed between 

 the size of the embryo at the time of expulsion from the egg 

 and the large sized ones which alone had been formerly ob- 

 served in the alimentary canal, and in this case we are almost 

 certain that the eggs are hatched in the alimentary canal, where 

 the embryos attain sexual maturity. 



The mode of development is usually slightly different in 

 Ascaris and Strongylus, where the eggs have a thin shell, and 

 the embryo enjoys for a time a non-parasitic existence in water 

 or mud. Here it grows in size, but develops no sexual organs. 

 Sometimes it accidentally attaches itself as a parasite to a fresh- 

 water mollusc, but it undergoes no change there. Eventually, 

 gaining access to a proper host, it reaches sexual maturity, and 

 the cycle of its life is repeated. This is the mode of develop- 

 ment of DocJwiius trigonocephalus of tlie dog, of DochmiiLS 

 duodenalis of man, and of various species of Ascaris, e.g., Ascaris 

 acuminata of the frog. In the case of some of the strongyles, 

 there can be no doubt that the earlier larval transformations 

 are undergone after the eggs have been expelled and lodged 

 within soft soil or mud. Leuckart has proved this in the case 

 of Strongylus hypostomus, whose rhabditiform young cast their 



