NEMATELMINTHES. 375 



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

 human parasites, Ascaris lumbricoides and Oxyurus 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 perma- 

 nently 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 become 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 duodenale under- 

 goes the same metamorphosis as Dochmius trigonocephalus. 



A remarkable modification of this type of metamorphosis is found in 

 Ascaris (Rhabdonema) 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 

 parent. 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 move- 

 ments 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 larvae eventually become 

 free, and though in many respects different from the parent form which gave 

 rise to them, have nevertheless the Rhabditis form. They live in water or 

 slime, and sometimes become parasitic in water-snails ; in neither case how- 

 ever do they undergo important changes unless eventually swallowed by a 

 frog. They then pass down the trachea into the lungs and there rapidly 

 develop into the adult form. No separate males have been found in the 

 lungs of the frog, but it has been shewn by Schneider (No. 390) that the 

 so-called females are really hermaphrodites ; the same gland giving origin 



