166 M. Agassiz on the Relations between Animals 



the remarkable result, that all these animals are aquatic, nay, 

 that, with one single exception, they are all marine. But before 

 this can be acknowledged, it must be shown that the type of Ra- 

 diata should be reduced to the three classes of Polypi, Jelly-fishes 

 and Echinoderms ; and that, among the Polypi, there are large 

 numbers of animals now united which do not all belong to that 

 class. The most extensive range acknowledged by some zoolo- 

 gists in the type of Radiata includes Infusoria, with the Rotifera, 

 and also intestinal worms. Without entering for the present 

 into a full discussion of the natural character of all the animals 

 which have been included in the class of Infusoria, I may limit 

 ray remarks to a few critical points, in order to show that the 

 Polygastrica, and even the Rotifera cannot be ranked among 

 Radiata. 



In the first place, the Rotifera constitute a particular group 

 among Infusoria, as Ehrenberg himself has acknowledged. They 

 differ so completely from the Polygastrica as to forbid entirely 

 their union in a natural classification. The only question is 

 whether they can remain among Radiata, and, if not, where they 

 should be placed. There is so little analogy between the struc- 

 ture of Rotifera and the structure of true Radiata, that ever since 

 the beautiful illustration of their forms and structure as given by 

 Ehrenberg, most naturalists and anatomists have felt inclined to 

 remove them to another type of the animal kingdom. Their re- 

 semblance to the Articulata has appeared to some so striking as 

 to warrant, in their opinion, their removal to the class of Crus- 

 tacea among Entomostraca, while others have considered them as 

 more closely allied to worms. But I may say that all, or almost 

 all, naturalists at present understand the necessity of removing 

 them from among Radiata into the great type of Articulata. 



This point is no longer in question ; the only remaining doubt 

 respecting them is whether they should rank among the lower 

 Crustacea or among the worms in the wider sense. As for the 

 Polygastrica, we meet with greater difiiculties in attempting to 

 classify them ; for this group, as understood by Ehrenberg, con- 

 sists still of most heterogeneous beings which do not even all 

 belong to the animal kingdom. Recent investigations upon the 

 so-called Anentera, including the families of Baccillaria and Vol- 

 vocine Infusoria, have satisfactorily shown, in my opinion, and in 

 that of most competent observers, that this type of Ehrenberg's 

 Polygastrica without gastric cavities, and without an elementary 

 tube, are really plants belonging to the order of Algae in the 

 widest extension of this group ; while most of the Monas tribe 

 are merely moveable germs of various kinds of other Algse. As 

 for the other Polygastrica which Ehrenberg combines in this di- 

 vision of Enterodela, I am satisfied that they also constitute still 



