OF THE RHIZOPODA IN GENERAL. 17 



tion in the threads of protoplasm. The low degree of differentiation in the protoplasmic 

 substance of these Rhizopods is manifested, not merely in their pseudopodia, but also in the 

 general mass of their bodies ; for although the external portion is less granular and coloured 

 than the internal, and is of somewhat firmer consistence, yet there is nothing like a definite 

 distinction between ectosarc and endosarc, so that the departure from the original homogeneity 

 of the sarcode is here reduced to its minimum. This low grade of differentiation is marked 

 also by the absence of the " nucleus" and of the " contractile vesicle," neither of which organs 

 has been yet detected in any members of this group. By far the larger proportion of the 

 Rhizopods which agree in these general characters are enclosed in calcareous shells, which 

 may be either monothalamous or polythalamous ; and such constitute the group of Forami- 

 nifera. In certain cases, however, the " test" is simply membranous (as in Gromia, Plate III, 

 fig. 2), or it may even be reduced to the condition of a very transparent film (as in Lieberkuhnia , 

 Plate II) ; and such forms are distinguished as Gromida. The physiological condition of all 

 these beings appears to be so closely accordant, as fully to justify their being combined in a 

 single Order, to which the name Reticulosa may be given, indicative of the reticulated 

 character of its pseudopodian extensions. The mode of subdividing this Order will be the 

 subject of discussion in the next Chapter. 



According to the foregoing views, the Rhizopoda may be thus arranged : 



RHIZOPODA. 



LoBosA. Radiolaria. Reticulosa. 



Amabina. Actvnopliryna. Gromida. 



/ \ Acanthometrina. Foratiiinifera. 



/ \ Folycystina. 



Thalassicollina. 



INFUSORIA. GREGARINIDA. SPONGIADA. 



PROTOPHYTA. 



