158 ANIMAL PARASITES AND MESSMATES. 



If the hermaphrodite Ascaris nigro-venosa alter- 

 nately produces individuals of separate sexes, that is to 

 say, if the monoecii produce dioecii, and the dioecii again 

 moncecii, one cannot help comparing this phenomenon to 

 digenetic generation. This is one of the striking dis- 

 coveries made at the laboratory of Giessen, under the 

 direction of Eud. Leuckart. Since then, Professor 

 Schneider, the successor of Leuckart at the University 

 of Giessen, has also studied these worms. Professor 

 Leuckart wrote thus to me a few days after this dis- 

 covery : " The Ascaris nigro-venosa presents this peculiar 

 phenomenon, that, under the parasitical form, it pro- 

 duces fertile eggs without the presence of males. The 

 embryos which proceed from the eggs become sexual 

 worms at the end of twenty-four hours after they have 

 left the body. This fact was first observed by M. 

 Mecznikow, while he was working in my laboratory, and 

 taking part in my researches. The experiment which 

 produced this result was suggested and directed by 

 myself, in order to continue my work on the develop- 

 ment of the Nematodes." 



We do not know if this is the place to Bpeak of an 

 animal which excited great attention some years ago, and 

 which was thought to prove the transformation of 

 animals into each other. It is a parasite which, under 

 the form of a gasteropod, lives under peculiar conditions. 

 It is known by the name of Entoconcha. Discovered by 

 J. Miiller in an echinoderm of the genus Synapta, its 

 complete development has been vainly sought to be 

 discovered since that time. It is evidently a gasteropod 

 mollusc, allied to the Natices, and lives in the interior 

 of the body of a Synapta, but we do not yet know all the 



