NEMATHELMIXTHES 243 



as one might imagine ; but the worms migrate iato a 

 Cyclops by first penetrating into the intestine thi'ough the 

 mouth and then into the body cavity of the Crustacean. 

 Here they undergo several changes in form, but attain their 

 permanent shape only after the Cyclops which harbors them 

 is swallowed by a perch, and they are set free in its 

 intestine, where they soon become sexually mature, and in 

 turn bring forth young, which undergo the same course of 

 development. JDracunculus rnedinensis, a Nematode parasitic 

 in the human body, appears to have a quite similar mode of 

 development. Bracuncidus inhabits the subdermal connect- 

 ive tissue, and by its pressure against the skin causes a 

 tumor and finally an abscess, through which it is able to 

 pass out. In this way also the embryos, which are present 

 in the woimi in countless numbers, may reach the outside 

 world. During the bathing of persons afflicted with the 

 disease they get into the water, and, like the larva3 of 

 Cucullanus, migrate into CyclopidaB ; however, they penetrate 

 directly through the body-covering to the interior of the 

 host. These infected Cyclopidse ai'e probably swallowed 

 along with the drinking-water by the inhabitants of those 

 regions where the parasite abounds. 



Spiroptera obtusa has a development very similar to that 

 of the two forms last considered, only it is still more 

 adapted to parasitic life, for the eggs of this worm do not 

 develop into a free organism, but are directly received by an 

 intermediate host. Spiroptera obtusa inhabits the intestine 

 of the mouse. The eggs, in which the embryos are already 

 developed, reach the outside world with the f feces. They 

 are swallowed by the larvse of the mealworm, Tenehrio, which 

 feed on the dung balls. The embryos hatch out in their 

 stomach, break through the wall of the intestine, and become 

 encysted in the fat-bodies of the mealworm. When a 

 mouse devours a mealworm, it becomes infected with 

 the Spiroptera, which wakes up to a new life in the intestine 

 of its host, becomes sexually mature and reproductive. 



The course of development in Trichina spiralis is one of 

 those which are most completely adapted to parasitic life, for 

 this Nematode accomplishes its entire life-history within the 



