DEVELOPMENT OF THE NEMATODES 273 



tion with the lymph stream. Some of them, no doubt, actively 

 bore through the intestinal wall, likewise attaining the lymph 

 stream or blood circulation, or even pass into the body cavity. 

 What occurs during their further migrations is difficult to say at 

 present. It has hitherto been maintained that the wandering is 

 entirely active ; for instance, the ligaturing of an artery would be 

 no protection against the part of the body supplied by such 

 artery being invaded by Trichinella. This observation cannot be 

 otherwise explained than by the active progress of the young Trichi- 

 nella. The question, however, may be mooted as to where and 

 when the worms quit the lymphatic system and the circulation 

 of the blood to wander further independently, and ultimately 

 reach the muscular system, in which they become encysted (fig. 

 190). Thus the progeny does not leave the body of the host 

 inhabited by the parents, as is generally the case amongst hel- 

 minths, but uses it as an intermediary carrier to reach another 

 host, which is then the final host. The latter may belong to 

 another species, or may be another individual of 'the same species. 

 This second migration is, of course, purely passive. 



In a number of Nematodes, however, HETEROGOXY occurs. 

 This term signifies that manner of development in which two 

 differently structured, sexual generations alternate with each other 

 within the same species. To these appertains, for instance, Rhab- 

 donema nigrovenosum, which lives in- the lungs of frogs and toads ; 

 this creature measures about i cm. in length and is hermaphroditic 

 (protandric). The eggs are deposited in the pulmonary cavity, and 

 through the cilia of the same reach the oral cavity, where they are 

 swallowed and thus conveyed into the intestine. They pass through 

 the entire gut, and are finally evacuated with the faeces ; often, indeed, 

 the young themselves emerge from the egg-shell within the hind gut 

 of the frogs. These young forms are sexually differentiated, remain 

 much smaller than the parent, their oesophagus is differently con- 

 structed (rhabditis form), and they are not parasitical (fig. 188). 

 After having grown in the open they copulate ; the males die off 

 soon after copulation, and the females in their own body develop 

 a few young, which, given the opportunity to get into frogs, settle 

 in them, and are transformed into hermaphroditic rhabdonema. The 

 same manner of development occurs in other species of the same 

 genus. 



Finally, it must be mentioned that some forms classified with 

 the Nematodes (Mermis, Gordius) are only parasitical during the 

 18 



