Structure of Rhizopod Shells. 311 



sideration as being employed by the Rhizopoda in the 

 construction of their shell are of threefold nature *. A 

 part of the Thalamophora construct their shells of 

 agglutinated foreign bodies, partly inorganic (sand, mud), 

 partly of oi-ganic nature (Thalaraophoran and Radiolarian 

 shells, sponge-spicules, &c.), while the greater part of the 

 Thalamophoran shells are formed by secretion of carbonate 

 of lime; and, thirdly, the skeletons of the E-adiolaria consist 

 of silica. The two first-mentioned materials of the Tha- 

 lamophoran shells have this in common, that they possess 

 far less firmness than the silicic acid of the Radiolaria. 

 This distinction has also as its consequence a corresponding 

 difference in the habit and mode of construction of the two 

 great primary groups of the Rhizopoda. 



Even upon a superficial examination one is struck with the 

 fact that the shells of the Thalamophora with much less 

 multiplicity of form and differentiation are far more massive 

 and stouter than the Radiolarian skeletons, which are often 

 exceedingly complicated, graceful, and elegant. The com- 

 paratively soft material which is employed by the Thalamo- 

 phora in the construction of their shells does not permit these 

 Rhizopoda without injuring the stability of their dwellings to 

 make such airy and complicated structures as the Radiolarian 

 skeletons, composed of solid more or less elastic siliceous 

 rods. 



The distinctions, however, are of a still more profound 

 nature, and extend not only to the external habit, but also to 

 the whole structural plan of the shells and skeletons. Even 

 in the single-shelled forms, this, as already indicated, may be 

 distinctly recognized in the selection or distribution of the 

 form-types in the two great sister-groups of the Rhizopoda. 

 The monothalamous Thalamophorous shells are almost all 

 pylomatic, and only a few forms, such as Orhulmella, Orhu- 

 lina^ and some sandy-shelled forms, belong to the perforate 

 form-type. On the other hand, among the Radiolaria the 



* The primary cliitinous shell of many freshwater Ehizopoda plays too 

 subordinate a part in the matters here under consideration to need any 

 special mention. The shell-material of the Phaeodarian families Circopo- 

 rida, Tuscarorida, and Challengerida requires closer investigation. It 

 appears, however, to have a similar consistency to the calcareous material 

 of the Thalamophoran shells, and the mode of construction of these Phoeo- 

 daria is like that of the Thalamophora. Acauthin appears, with regard 

 to its soUdity, to hold a middle place between carbonate of lime and 

 silicic acid, at least this holds with regard to the habit of the Acantharian 

 skeletons, which, on the one hand, are more differentiated and elegant 

 than the Thalamophoran shells, without, however, on the other, attaining 

 the light construction and great complication of the siliceous skeletons of 

 the Polycystina and Phaeodaria. 



