Animals and the Elements in which they Live, 197 



entering, for the present, into a full discussion of the natural 

 trharacter of all the animals which have been included in the 

 class of Infusoria, I may limit my remarks to a few critical 

 points, in order to show that the Polygastrica, -and even the 

 Rotifera, cannot be ranked among the Radiata. 



In the first place, Rotifera constitutes 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 

 <iot, where they should be placed. There is so little analogy 

 between the structure of Rotifera and the structure of true Ra- 

 diata, 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 resemblance to Articu- 

 lata has appeared to some so striking as to warrant, in their 

 opinion, their removal to the class of Crustacea, 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 remov- 

 ing them from among Radiata into the great type of Articu- 

 lata. 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, consists still of most heterogeneous beings, 

 which do not even all belong to the animal kingdom. Recent 

 investigatio;is upon the so-called Anentera, including the 

 families of Baccillaria and Volvocine Infusoria, have satis- 

 factorily shewn, in my opinion, and in that of most com- 

 petent observers, that this type of Ehrenberg's Polygastrica 

 without gastric cavities, and without an elementary tube, are 

 really plants belonging to the order of AlgsB, in the widest 

 extension of this group ; whilst most of the Monas tribe are 

 merely movable germs of various kinds of other Algae. As 

 for the other Polygastrica, which Ehrenberg combines in this 

 division of Enterodela, I am satisfied that they also consti- 

 tute still a heterogeneous group belonging to different types 



