Structure of RMzopod Shells. 317 



isomorphous arenaceous and calcareous forms, occurs, how- 

 ever, only in Thalamophora moderately high in development, 

 and, indeed, is possible only in them because here the corre- 

 sponding morphological change extends only to unimportant 

 peculiarities, but impossible in the highest and most differ- 

 entiated types, such as the Nummulites for example, in which 

 a reversion to the arenaceous grade of development would 

 need to be accompanied by a profound change in the whole 

 structure of the shell. 



Thus, then, we have seen that in the three principal 

 materials which come under consideration in connexion with 

 the hard structures of the Khizopoda, so many degrees of 

 firmness and fineness may be recognized, which exert a very 

 considerable influence upon the structure of the shells and 

 skeletons. If we would illustrate these conditions by an 

 example out of everyday life, we may fairly compare the 

 agglutinated arenaceous material, the carbonate of lime, and 

 the silicic acid, as the materials of the Rhizopod shells, on the 

 one hand, with mud, stone, and iron, the three most important 

 substances in the buildings made by man. Tiie mud-struc- 

 tures, like the arenaceous Uhizopod-shells, can be carried out 

 only in a rough and more or less primitive manner, like the 

 birds' nests (such as those of the Swallows) built of mud, 

 owing to the coarse texture and want of solidity in the 

 material employed ; in fact, the mutually adherent chambers 

 of many " Agglutinantia " among the Rhizopoda possess a 

 remarkable resemblance to the Swallows' nests aggregated 

 together on the wall of a house. Stone- buildings and the 

 calcareous Rhizopod shells take an intermediate position ; 

 while the siliceous skeletons of the Radiolaria and the infi- 

 nitely varied iron structures of everyday life, from the great 

 solidity of the materials, give the greatest room for compli- 

 cation and differentiation, and at the same time for multipli- 

 city of form. It is not only the inherited faculty of the soft 

 body to construct more or less complicated and differentiated 

 skeletal parts that regulates the shell-structure, but, like 

 human architects, the Rhizopoda are also more or less depen- 

 dent upon their material, and must deal with its peculiarities. 



As we have already seen, the concentric growth makes 

 greater demands than the terminal upon the firmness of the 

 material, and it is therefore met with only in the siliceous 

 Radiolarian skeletons, while it does not occur among the 

 Thalamophora. But at the same time the concentric skeletal 

 structure has an advantage of which the terminal is destitute. 

 A system of several nested spherical shells or parts of such 

 shells forms an externally closed rounded whole which presents 



