MODERN SYSTEMS. 291 



others. Indeed, they are, in my opinion, the only inde- 

 pendent animals of that group ; and, so far from having 

 any natural affinity with the other Enterodela, I do not 

 doubt that their true place is by the side of the Bryozoa, 

 among the Mollusks, as I shall attempt to show presently. 

 Isolated observations which I have been able to make 

 upon Paramecium, Opalina, and the like, seem to me 

 sufficient to justify the assumption that they disclose the 

 true nature of the bulk of this group. I have seen, for 

 instance, a Planaria lay eggs out of which Paramecia 

 were born, which underwent all the changes these animals 

 are known to undergo up to the time of their contraction 

 into a chrysalis state ; while the Opalina is hatched from 

 Distoma's eggs. I shall publish the details of these ob- 

 servations on another occasion. But if it can be shown 

 that two such types as Paramecium and Opalina are the 

 progeny of Worms, it seems to me to follow that all the 

 Enterodela, with the exception of the Vorticellidse, must 

 be considered as the embryonic condition of that host of 

 Worms, both parasitic and free, the metamorphosis of 

 which is still unstudied. In this connection I might 

 further remark, that the time is not long past when 

 Cercaria was also considered as belonging to the class of 

 Infusoria, though at present no one doubts that it belongs 

 to the cycle of Distoma ; and the only link in the me- 

 tamorphosis of that genus which was not known is now 

 supplied, since, as I have stated above, the embryo which 

 is hatched from the egg laid by the perfect Distoma is 

 found to be Opalina. 



All this leads to the conclusion, that a division of the 

 animal kingdom to be called Protozoa, differing from all 

 other animals in producing no eggs, does not exist in 

 nature, and that the beings which have been referred to 



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