310 METAMORPHOSIS. 



In the first group with- one host the simplest cases are those in 

 which the adult sexual form of parasite lays its eggs in the alimentary 

 tract of its host, and the eggs are thence transported to the exterior. 

 The embryo still in the egg, if favoured by sufficient warmth and 

 moisture, completes its development up to a certain point, and, if then 

 swallowed by an individual of the species in which it is parasitic in 

 the adult condition, it is denuded of its shell by the action of the 

 gastric juice, and develops directly into the sexual form. 



Leuckart has experimentally established this metamorphosis in the 

 case of Trichocephalus affinis, Oxyurus ambigua, and Heterakis vermicularis. 

 The Oxyuridse of Blatta and Hyclropkilus have a similar life histoiy 

 (Galeb, No. 386), and it is almost certain that the metamorphosis of the 

 human parasites, Ascaris lumbricoides and Oxym-us vermicularis, is of this 

 nature. 



A slightly more complicated metamorphosis is common in the 

 genera Ascaris and Strongylus. In these cases the egg-shell is thin, 

 and the embryo becomes free externally, and enjoys for a shorter or 

 longer period a free existence in water or moist earth. During this 

 period it grows in size, and though not sexual usually closely resembles 

 the adult form of the permanently free genus Rhabditis. In some 

 cases the free larva becomes parasitic in a freshwater Mollusc, but 

 without thereby undergoing any change. It eventually enters the 

 alimentary tract of its proper host and there becomes sexual. 



As examples of this form of development worked out by Leuckart may 

 be mentioned Dochmius trigonocephalus, parasitic in the dog, and Ascaris 

 acuminata, in the frog. The human parasite Dochmius duodeiiale under- 

 goes the fame metamorphosis as Dochmius trigonocephalus. 



A remarkable modification of this type of metamorphosis is found in 

 Ascaris (Rkabdonema) nigrovenosa, which in its most developed condition 

 is parasitic in the lungs of the frog (Metschnikoff, Leuckart, No. 388). 

 The embryos pass through their first developmental phases in the body of 

 the pai-ent. They have the typical Rhabditis form, and make their way 

 after birth into the frog's rectum. From this they pass to the exterior, 

 and then living either in moist earth, or the faeces of the frog, develop 

 into a sexual form, but are very much smaller than in the adult condition. 

 The sexes are distinct, and the males are distinguished from the females by 

 their smaller size, shorter and rounded tails, and thinner bodies. The 

 females have paired ovaries with a very small number of eggs, but the 

 testis of the males is unpaired. Impregnation takes place in the usual way, 

 and in summer time about four embryos are developed in each female, which 

 soon burst their egg-capsules, and then move freely in the uterus. Their 

 active movements soon burst the uterine walls, and they then come to lie 

 freely in the body cavity. The remaining viscera of the mother are next 

 reduced to a finely granular material, which serves for the nutrition of 

 the young forms which continue to live in the maternal skin. The larvaj 

 eventually become free, and though in many respects different from the 

 parent form which gave rise to them, have nevertheless tlie Rhabditis form. 

 They live in water or slime, and sometimes become parasitic in water-snails; 



