242 THE AMERICAN MONTHLY [October, 



plasm itself which the metabolic function is constantly exerted to 

 make, and such forms as Hyalosphenia and others would, on 

 this ground as well as others, have to be considered more prim- 

 itive. The production of a skeleton would make movement 

 unnecessary and perhaps inconvenient, and hence the pseudo- 

 podia would be more likely to be specialized as food collectors 

 than as organs of locomotion in Protozoa with a skeleton, and 

 such we find them in the Rhizopoda. The specialization of the meta- 

 bolic power in secreting a skeleton could not take place on a large 

 number of conceivable lines, the commonest stony material being 

 more likely to be used, and the lighter and stronger ones ; in point 

 of fact, only two minerals are used in skeleton-building, namely, 

 lime and silica. We suppose that the secretion of lime in Rotalia 

 and its deposition in the body is as definite an act as the move- 

 ment of a flagellum, and that it is as subject to the laws of improve- 

 ment by exercise, heredity, etc. The secretion of a membrane- 

 ous skeleton takes place in both the Foraminifera and the Heliozoa, 

 but the more highly specialized members of the orders have a 

 skeleton of lime on the one hand or silica on the other. 



Specialization of motility, on the other hand, would make a pro- 

 tective skeleton less necessary, and the specialization of a definite 

 mouth and the loss of the food-gathering function of pseudopodia 

 would put a premium on motion, for an active animal would have 

 more chances of finding food. With activity constant, stimulation 

 would result and thus the possibility of heightened sensitiveness 

 or improved power of sensation would come. The uses of the 

 pseudopodia as flagella or cilia are also cases of varied develop- 

 ment of powers, and the varieties of ciliation are cases of the 

 same principle. When the Protozoa are fully understood it is 

 hoped that we can indicate the steps of development which have 

 led up from a shapeless and indefinite lump of protoplasm to the 

 exceedingly complete and perfect organization of Vorticella. It 

 is very interesting to find that some of the Infusoria, e.g., Pyxicola, 

 have a sort of skeleton and that it is membraneous, the lowest kind 

 of skeleton. The advantages of combination are only slightly 

 secured by the Protozoa, if at all. But it is very interesting to find 

 colonies of members in this group, because of the immense im- 

 portance of cell combination in body construction in higher phyla. 

 Here, as so often, we find an example of a sort of prophetic antici- 

 pation in lower groups of the conditions which are fundamental 

 in higher groups. In Codosiga the mechanical union of the cells 

 is difficult to explain in terms of advantage. 



While two classes thus result from specializations in metabolic 

 or motor powers, a third in which these are at a minimum is pos- 

 sible, provided the need of their exercise is removed, and in a 

 parasitic existence such need would be gone. 



The sporozoa are such a class. No one has ever dared to state 

 a conviction whether the sporozoa are probable degenerate de- 

 scendants from Rhizopoda or Infusoria, but they are generally un- 

 derstood to be from the same ancestry as one or other of these. 



